More accurately report text parsing failures for invalid strings.

proto/text_parser.go

@@ -174,7 +174,7 @@ func (p *textParser) advance() {
 		}
 		unq, err := unquoteC(p.s[1:i], rune(p.s[0]))
 		if err != nil {
-			p.errorf("invalid quoted string %v", p.s[0:i+1])
+			p.errorf("invalid quoted string %s: %v", p.s[0:i+1], err)
 			return
 		}
 		p.cur.value, p.s = p.s[0:i+1], p.s[i+1:len(p.s)]

proto/text_parser_test.go

@@ -152,7 +152,7 @@ var unMarshalTextTests = []UnmarshalTextTest{
 	// Bad quoted string
 	{
 		in:  `inner: < host: "\0" >` + "\n",
-		err: `line 1.15: invalid quoted string "\0"`,
+		err: `line 1.15: invalid quoted string "\0": \0 requires 2 following digits`,
 	},
 
 	// Number too large for int64