function/stdlib: Consistent csvdecode error messages

Go 1.17 changes the way the CSV parser reports columns in its error
objects, causing our error results to be inconsistent depending on which
Go version we're built with.

In order to make the error messages consistent we'll use our own custom
error formatter for that case. This does mean we lose the column number
in the error message, but we'll still include the line number where the
problem was detected.

cty/function/stdlib/csv.go

@@ -30,7 +30,7 @@ var CSVDecodeFunc = function.New(&function.Spec{
 			return cty.DynamicPseudoType, fmt.Errorf("missing header line")
 		}
 		if err != nil {
-			return cty.DynamicPseudoType, err
+			return cty.DynamicPseudoType, csvError(err)
 		}
 
 		atys := make(map[string]cty.Type, len(headers))
@@ -64,7 +64,7 @@ var CSVDecodeFunc = function.New(&function.Spec{
 				break
 			}
 			if err != nil {
-				return cty.DynamicVal, err
+				return cty.DynamicVal, csvError(err)
 			}
 
 			vals := make(map[string]cty.Value, len(cols))
@@ -91,3 +91,12 @@ var CSVDecodeFunc = function.New(&function.Spec{
 func CSVDecode(str cty.Value) (cty.Value, error) {
 	return CSVDecodeFunc.Call([]cty.Value{str})
 }
+
+func csvError(err error) error {
+	switch err := err.(type) {
+	case *csv.ParseError:
+		return fmt.Errorf("CSV parse error on line %d: %w", err.Line, err.Err)
+	default:
+		return err
+	}
+}

cty/function/stdlib/csv_test.go

@@ -58,7 +58,7 @@ func TestCSVDecode(t *testing.T) {
 		{
 			cty.StringVal(`invalid"thing"`),
 			cty.DynamicVal,
-			`parse error on line 1, column 7: bare " in non-quoted-field`,
+			`CSV parse error on line 1: bare " in non-quoted-field`,
 		},
 		{
 			cty.UnknownVal(cty.String),