Fix formatting of wrapped errors when colors are used

There are two different code paths for rendering a key/value pair. The
non-color version uses a type switch that handles specific types such as
"error", and the color version uses the %+v printf format specifier.
This causes an inconsistency between the two formats. In particular,
errors created using the github.com/pkg/errors package will include a
stack trace of where the error was created when printed to the terminal,
but not to a file. Printing the stack trace as part of the log field is
probably not the right behavior.

The output is also inconsistent between the two forms because strings
are not quoted/escaped when colors are used. This can make log output
unparseable.

Fix this by making both code paths use the type switch and escaping
rules. Fix the escaping code to pass the error value to Fprintf, not the
error itself, which seems to be necessary to avoid blank output with
errors created by github.com/pkg/errors.

text_formatter.go

@@ -122,7 +122,8 @@ func (f *TextFormatter) printColored(b *bytes.Buffer, entry *Entry, keys []strin
 	}
 	for _, k := range keys {
 		v := entry.Data[k]
-		fmt.Fprintf(b, " \x1b[%dm%s\x1b[0m=%+v", levelColor, k, v)
+		fmt.Fprintf(b, " \x1b[%dm%s\x1b[0m=", levelColor, k)
+		f.appendValue(b, v)
 	}
 }
 
@@ -142,7 +143,11 @@ func (f *TextFormatter) appendKeyValue(b *bytes.Buffer, key string, value interf
 
 	b.WriteString(key)
 	b.WriteByte('=')
+	f.appendValue(b, value)
+	b.WriteByte(' ')
+}
 
+func (f *TextFormatter) appendValue(b *bytes.Buffer, value interface{}) {
 	switch value := value.(type) {
 	case string:
 		if !needsQuoting(value) {
@@ -155,11 +160,9 @@ func (f *TextFormatter) appendKeyValue(b *bytes.Buffer, key string, value interf
 		if !needsQuoting(errmsg) {
 			b.WriteString(errmsg)
 		} else {
-			fmt.Fprintf(b, "%q", value)
+			fmt.Fprintf(b, "%q", errmsg)
 		}
 	default:
 		fmt.Fprint(b, value)
 	}
-
-	b.WriteByte(' ')
 }