v2: errors (#534)

```
name                              old time/op    new time/op    delta
UnmarshalDataset/config-32          86.7ms ± 2%    87.5ms ± 2%     ~     (p=0.113 n=9+10)
UnmarshalDataset/canada-32           129ms ± 4%     106ms ± 3%  -17.94%  (p=0.000 n=10+10)
UnmarshalDataset/citm_catalog-32    59.4ms ± 5%    58.7ms ± 5%     ~     (p=0.393 n=10+10)
UnmarshalDataset/twitter-32         27.0ms ± 7%    26.9ms ± 6%     ~     (p=0.720 n=10+9)
UnmarshalDataset/code-32             326ms ± 4%     322ms ± 7%     ~     (p=0.661 n=9+10)
UnmarshalDataset/example-32          510µs ±11%     526µs ± 7%     ~     (p=0.182 n=10+9)
UnmarshalSimple-32                  1.41µs ± 6%    1.41µs ± 4%     ~     (p=0.736 n=10+9)
ReferenceFile-32                    45.6µs ± 3%    43.9µs ±10%     ~     (p=0.089 n=10+10)

name                              old speed      new speed      delta
UnmarshalDataset/config-32        12.1MB/s ± 2%  12.0MB/s ± 2%     ~     (p=0.108 n=9+10)
UnmarshalDataset/canada-32        17.1MB/s ± 4%  20.9MB/s ± 3%  +21.86%  (p=0.000 n=10+10)
UnmarshalDataset/citm_catalog-32  9.41MB/s ± 5%  9.51MB/s ± 5%     ~     (p=0.362 n=10+10)
UnmarshalDataset/twitter-32       16.4MB/s ± 8%  16.5MB/s ± 6%     ~     (p=0.704 n=10+9)
UnmarshalDataset/code-32          8.24MB/s ± 4%  8.34MB/s ± 7%     ~     (p=0.675 n=9+10)
UnmarshalDataset/example-32       15.9MB/s ±11%  15.4MB/s ± 7%     ~     (p=0.182 n=10+9)
ReferenceFile-32                   115MB/s ± 4%   120MB/s ±10%     ~     (p=0.085 n=10+10)

name                              old alloc/op   new alloc/op   delta
UnmarshalDataset/config-32          16.9MB ± 0%    16.9MB ± 0%   -0.02%  (p=0.000 n=10+10)
UnmarshalDataset/canada-32          76.8MB ± 0%    74.3MB ± 0%   -3.31%  (p=0.000 n=10+10)
UnmarshalDataset/citm_catalog-32    37.3MB ± 0%    37.1MB ± 0%   -0.60%  (p=0.000 n=9+10)
UnmarshalDataset/twitter-32         15.6MB ± 0%    15.6MB ± 0%   -0.09%  (p=0.000 n=10+10)
UnmarshalDataset/code-32            60.2MB ± 0%    59.3MB ± 0%   -1.51%  (p=0.000 n=10+9)
UnmarshalDataset/example-32          238kB ± 0%     238kB ± 0%   -0.18%  (p=0.000 n=10+10)
ReferenceFile-32                    11.8kB ± 0%    11.8kB ± 0%     ~     (all equal)

name                              old allocs/op  new allocs/op  delta
UnmarshalDataset/config-32            653k ± 0%      645k ± 0%   -1.20%  (p=0.000 n=10+6)
UnmarshalDataset/canada-32           1.01M ± 0%     0.90M ± 0%  -11.04%  (p=0.000 n=9+10)
UnmarshalDataset/citm_catalog-32      384k ± 0%      370k ± 0%   -3.75%  (p=0.000 n=10+10)
UnmarshalDataset/twitter-32           160k ± 0%      157k ± 0%   -1.32%  (p=0.000 n=10+10)
UnmarshalDataset/code-32             2.97M ± 0%     2.91M ± 0%   -2.15%  (p=0.000 n=10+7)
UnmarshalDataset/example-32          3.69k ± 0%     3.63k ± 0%   -1.52%  (p=0.000 n=10+10)
ReferenceFile-32                       253 ± 0%       253 ± 0%     ~     (all equal)
```

scanner.go

@@ -1,9 +1,5 @@
 package toml
 
-import (
-	"errors"
-)
-
 func scanFollows(b []byte, pattern string) bool {
 	n := len(pattern)
 
@@ -83,22 +79,17 @@ func scanMultilineLiteralString(b []byte) ([]byte, []byte, error) {
 	return nil, nil, newDecodeError(b[len(b):], `multiline literal string not terminated by '''`)
 }
 
-var (
-	errWindowsNewLineMissing = errors.New(`windows new line missing \n`)
-	errWindowsNewLineCRLF    = errors.New(`windows new line should be \r\n`)
-)
-
 func scanWindowsNewline(b []byte) ([]byte, []byte, error) {
-	const lenLF = 2
-	if len(b) < lenLF {
-		return nil, nil, errWindowsNewLineMissing
+	const lenCRLF = 2
+	if len(b) < lenCRLF {
+		return nil, nil, newDecodeError(b, "windows new line expected")
 	}
 
 	if b[1] != '\n' {
-		return nil, nil, errWindowsNewLineCRLF
+		return nil, nil, newDecodeError(b, `windows new line should be \r\n`)
 	}
 
-	return b[:lenLF], b[lenLF:], nil
+	return b[:lenCRLF], b[lenCRLF:], nil
 }
 
 func scanWhitespace(b []byte) ([]byte, []byte) {
@@ -116,8 +107,6 @@ func scanWhitespace(b []byte) ([]byte, []byte) {
 
 //nolint:unparam
 func scanComment(b []byte) ([]byte, []byte) {
-	// ;; Comment
-	//
 	// comment-start-symbol = %x23 ; #
 	// non-ascii = %x80-D7FF / %xE000-10FFFF
 	// non-eol = %x09 / %x20-7F / non-ascii
@@ -132,10 +121,6 @@ func scanComment(b []byte) ([]byte, []byte) {
 	return b, nil
 }
 
-var errBasicLineNotTerminatedByQuote = errors.New(`basic string not terminated by "`)
-
-//nolint:godox
-// TODO perform validation on the string?
 func scanBasicString(b []byte) ([]byte, []byte, error) {
 	// basic-string = quotation-mark *basic-char quotation-mark
 	// quotation-mark = %x22            ; "
@@ -156,11 +141,9 @@ func scanBasicString(b []byte) ([]byte, []byte, error) {
 		}
 	}
 
-	return nil, nil, errBasicLineNotTerminatedByQuote
+	return nil, nil, newDecodeError(b[len(b):], `basic string not terminated by "`)
 }
 
-//nolint:godox
-// TODO perform validation on the string?
 func scanMultilineBasicString(b []byte) ([]byte, []byte, error) {
 	// ml-basic-string = ml-basic-string-delim [ newline ] ml-basic-body
 	// ml-basic-string-delim