Fix %q format for wrapped errors (#58)

Not handling the %q format causes logrus to log empty error strings for
wrapped errors where x.Error() contains spaces or other quoteworthy
characters.

For reference, the logrus formatter does this:
https://github.com/Sirupsen/logrus/blob/master/text_formatter.go#L154

errors.go

@@ -152,6 +152,8 @@ func (w wrapper) Format(s fmt.State, verb rune) {
 		fallthrough
 	case 's':
 		io.WriteString(s, w.Error())
+	case 'q':
+		fmt.Fprintf(s, "%q", w.Error())
 	}
 }
 

format_test.go

@@ -83,6 +83,10 @@ func TestFormatWrap(t *testing.T) {
 		Wrap(io.EOF, "error"),
 		"%s",
 		"error: EOF",
+	}, {
+		Wrap(New("error with space"), "context"),
+		"%q",
+		`"context: error with space"`,
 	}}
 
 	for _, tt := range tests {
@@ -109,7 +113,7 @@ func TestFormatWrapf(t *testing.T) {
 		"EOF\n" +
 			"error\n" +
 			"github.com/pkg/errors.TestFormatWrapf\n" +
-			"\t.+/github.com/pkg/errors/format_test.go:107",
+			"\t.+/github.com/pkg/errors/format_test.go:111",
 	}, {
 		Wrapf(New("error"), "error%d", 2),
 		"%v",
@@ -119,14 +123,14 @@ func TestFormatWrapf(t *testing.T) {
 		"%+v",
 		"error\n" +
 			"github.com/pkg/errors.TestFormatWrapf\n" +
-			"\t.+/github.com/pkg/errors/format_test.go:118",
+			"\t.+/github.com/pkg/errors/format_test.go:122",
 	}, {
 		Wrap(Wrap(io.EOF, "error1"), "error2"),
 		"%+v",
 		"EOF\n" +
 			"error1\n" +
 			"github.com/pkg/errors.TestFormatWrapf\n" +
-			"\t.+/github.com/pkg/errors/format_test.go:124\n",
+			"\t.+/github.com/pkg/errors/format_test.go:128\n",
 	}}
 
 	for _, tt := range tests {