Improve logging of API errors. (#553)
Improve logging of API errors. (#553)

Previously errors would look like:
ERRO[0008] api error: {bad_data 0xc4201caae0}
source=api.go:534

Now they look like:
ERRO[0001] api error: bad_data: json: cannot unmarshal object into Go
value of type []*types.Alert  source=api.go:534
tinytub/alertmanager,brunoqc/alertmanager,GoogleCloudPlatform/alertmanager,Aloomaio/alertmanager,Kellel/alertmanager,brunoqc/alertmanager,tyrken/alertmanager,tyrken/alertmanager,tinytub/alertmanager,tyrken/alertmanager,prometheus/alertmanager,joehandzik/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,Kellel/alertmanager,brunoqc/alertmanager,tyrken/alertmanager,tinytub/alertmanager,Aloomaio/alertmanager,Aloomaio/alertmanager,joehandzik/alertmanager,prometheus/alertmanager,joehandzik/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,brunoqc/alertmanager,Kellel/alertmanager