dvc-cml must stop if no credentials found or error in pull

[DavidGOrtega]

from #47

ERROR: unexpected error - Unable to locate credentials   <--- THIS IS IN RED
[EXIT NOW - NO REASON TO CONTINUE]
##[error]Process completed with exit code 1.               <--- THIS IS IN RED 

[dmpetrov]

@DavidGOrtega please make sure it stops only if pull when pull is not suppressed #49

[DavidGOrtega]

yes, I have to control also one scenario that I added an issue in dvc

// Only throw if not credentials since it may throw if never ran repro before
    // or won't have tracked folders
    // https://github.com/iterative/dvc/issues/3433

but Im actually rechecking this also