Skip func type value in fields.

We skip those unprintable fields and an error field
instead of dropping the whole trace.

Fixes #642

formatter.go

@@ -2,7 +2,14 @@ package logrus
 
 import "time"
 
-const defaultTimestampFormat = time.RFC3339
+// Default key names for the default fields
+const (
+	defaultTimestampFormat = time.RFC3339
+	FieldKeyMsg            = "msg"
+	FieldKeyLevel          = "level"
+	FieldKeyTime           = "time"
+	FieldKeyLogrusError    = "logrus_error"
+)
 
 // The Formatter interface is used to implement a custom Formatter. It takes an
 // `Entry`. It exposes all the fields, including the default ones:
@@ -48,4 +55,10 @@ func prefixFieldClashes(data Fields, fieldMap FieldMap) {
 		data["fields."+levelKey] = l
 		delete(data, levelKey)
 	}
+
+	logrusErrKey := fieldMap.resolve(FieldKeyLogrusError)
+	if l, ok := data[logrusErrKey]; ok {
+		data["fields."+logrusErrKey] = l
+		delete(data, logrusErrKey)
+	}
 }