Improve the quality of tag parsing errors.

tag_test.go

@@ -121,7 +121,7 @@ func TestParseTag(t *testing.T) {
 		opts, err := parseTag(test.input)
 		assert.Equal(t, test.opts, opts)
 		if err != nil {
-			assert.Equal(t, test.errstr, err.Error())
+			assert.Contains(t, err.Error(), test.errstr)
 		}
 	}
 }

typestr.go

@@ -1,13 +1,14 @@
 package restruct
 
 import (
-	"errors"
 	"fmt"
 	"go/ast"
 	"go/parser"
 	"go/token"
 	"reflect"
 	"strconv"
+
+	"github.com/pkg/errors"
 )
 
 // typeMap maps identifiers to reflect.Types.
@@ -99,7 +100,7 @@ func typeOfExpr(expr ast.Expr) (reflect.Type, error) {
 func parseType(typ string) (reflect.Type, error) {
 	expr, err := parser.ParseExpr(typ)
 	if err != nil {
-		return nil, errors.New("parsing error")
+		return nil, errors.Wrap(err, "parsing error")
 	}
 
 	return typeOfExpr(expr)

typestr_test.go

@@ -78,7 +78,7 @@ func TestParseType(t *testing.T) {
 			assert.Equal(t, test.typ.String(), typ.String())
 		}
 		if err != nil {
-			assert.Equal(t, test.errstr, err.Error())
+			assert.Contains(t, err.Error(), test.errstr)
 		}
 	}
 }