fix step yml parsing error log

[godrei]

From this:

ERRO[16:35:58] Step (local step) failed, error: yaml: unmarshal errors:
  line 2: cannot unmarshal !!map into string

to this:

ERRO[16:36:29] Step (local step) failed: failed to parse step definition ($HOME/Develop/go/src/github.com/bitrise-steplib/steps-xcode-test/step.yml): yaml: unmarshal errors:
  line 2: cannot unmarshal !!map into string

[richard-bogdan-bitrise]

Review ok.