Using WithError for Warn levels and below behaves weirdly

[fizyk]

When using WithError alog with Warn and below, the formatter leaves the error key in data, but strips the error message leaving an empty string.

logrus.WithError(errors.New("test")).Warn("Issues!")

{"timestamp":"2009-11-10T23:00:00Z","message":"Issues !","severity":"WARNING","context":{"data":{"error":{}}}}

logrus.WithError(errors.New("test")).Error("Issues!")

{"timestamp":"2009-11-10T23:00:00Z","serviceContext":{},"message":"Issues!: test","severity":"ERROR","context":{"reportLocation":{"filePath":"sandbox634680707/prog.go","lineNumber":21,"functionName":"main"}}}

Console output

[DamianSkrzypczak]

I checked and problem lays in different behavior for certain levels (severityError, severityCritical, severityAlert) (https://github.com/TV4/logrus-stackdriver-formatter/blob/master/formatter.go#L150)
Those threat error field specially.

For any other logging level, any error-typed value is just passed within context object (context.Data to be specific) to json.Marshal (https://github.com/TV4/logrus-stackdriver-formatter/blob/master/formatter.go#L187) which just ignores error-typed values (but keeps keys)

example of such behaviour can be found here:
https://play.golang.org/p/pVFYjn1BoL7

So potential fix would be to find all error-typed values and convert them into string using .Error() or require from user to use MarshalJSON()-able objects that fulfill error interface as well.
(example of such custom error https://play.golang.org/p/Z7YNNl6lNZE)

Example of proposed custom type working with logrus.WithError(...).Warn(...) can be found here: https://play.golang.org/p/wOziKs36nRU