RFE: Provide a clear error message if the config references a branch that does not exist

[rpbarlow-credible]

Greetings!

In #33, I encountered a strange error message when I used a branch name that didn't exist in my repo in my gitflow-action configuration:

##[error]Validation Failed: {"resource":"PullRequest","field":"base","code":"invalid"}

I suggest detecting this condition and reporting a clearer error message. Something similar to this would be helpful:

Error: Setting 'dev' references a branch that does not exist: devel

[Logerfo]

@rpbarlow thank you.