Improve error message when wrong repo is entered

[willingc]

Use case

• I enter a repo (i.e. willingc/think-dsp) in the "repo or URL" box.
• I press launch.
• The build log opens and tells me to check the URL.
• I'm confused since I entered a repo and not a URL.
• I look at the URL to share which looks correct at quick glance to me.
• I enter repo again and press launch. Same results. I finally realize that I have spelled the repo incorrectly and reenter.

Suggestion

Improve error message to offer user more guidance to check the repo or URL entered.

Perhaps instead of "Could not resolve ref for gh:willingc/think-dsp/master. Double check your URL."

"Could not resolve a reference to gh:willingc/think-dsp/master. Double check your entry for repo or URL."

[delmeyer]

I second the suggestion of @willingc - I'm really impressed with the MyBinder functionality, documentation and 'examples' gallery, but the most important feature of simply launching a (github) repo needs improvement in my opinion. The error messaging is definitely lacking...'double-check your url' seems to be too vague of a suggestion in trying to 'resolve' the issue. Please consider improving the error messaging for users.