gitlab_project_variable: friendlier error when masked variable is invalid

[armsnyder]

Closes #371

Rather than adding validation I intercepted the error.

Old error:

Error: POST https://gitlab.com/api/v4/projects/12345678/variables: 400 {message: {value: [is invalid]}}

New error:

Error: Invalid value for a masked variable. Check the masked variable requirements: https://docs.gitlab.com/ee/ci/variables/#masked-variable-requirements

[michaelwooley]

Can't speak to the wider code quality, etc. However, the message/change looks like it should save some people a few headaches! Thanks for the turnaround.

[armsnyder]

@nicholasklick This one should ready to merge.