time: fix error message from Parse

Was incorrectly discarding the offending text in some cases.

Fixes #4493.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7277050

src/pkg/time/format.go

@@ -611,14 +611,14 @@ func skip(value, prefix string) (string, error) {
 	for len(prefix) > 0 {
 		if prefix[0] == ' ' {
 			if len(value) > 0 && value[0] != ' ' {
-				return "", errBad
+				return value, errBad
 			}
 			prefix = cutspace(prefix)
 			value = cutspace(value)
 			continue
 		}
 		if len(value) == 0 || value[0] != prefix[0] {
-			return "", errBad
+			return value, errBad
 		}
 		prefix = prefix[1:]
 		value = value[1:]

src/pkg/time/time_test.go

@@ -676,6 +676,11 @@ var parseErrorTests = []ParseErrorTest{
 	// issue 4502. StampNano requires exactly 9 digits of precision.
 	{StampNano, "Dec  7 11:22:01.000000", `cannot parse ".000000" as ".000000000"`},
 	{StampNano, "Dec  7 11:22:01.0000000000", "extra text: 0"},
+	// issue 4493. Helpful errors.
+	{RFC3339, "2006-01-02T15:04:05Z07:00", `parsing time "2006-01-02T15:04:05Z07:00": extra text: 07:00`},
+	{RFC3339, "2006-01-02T15:04_abc", `parsing time "2006-01-02T15:04_abc" as "2006-01-02T15:04:05Z07:00": cannot parse "_abc" as ":"`},
+	{RFC3339, "2006-01-02T15:04:05_abc", `parsing time "2006-01-02T15:04:05_abc" as "2006-01-02T15:04:05Z07:00": cannot parse "_abc" as "Z07:00"`},
+	{RFC3339, "2006-01-02T15:04:05Z_abc", `parsing time "2006-01-02T15:04:05Z_abc": extra text: _abc`},
 }
 
 func TestParseErrors(t *testing.T) {