frontend: return a user friendly error when modulemd is not ok
    
    Fix #1382
    
    Instead of a typical last-resort exception message
    
        Error: Request wasn't successful, there is probably a bug in the API code
    
    Print something more user-frinedly, e.g.
    
        Error: Unsupported or malformed modulemd yaml - Missing required option
        data.summary
    
        Error: Invalid modulemd yaml - Provided YAML did not begin with
        a module document
    
        Error: Invalid modulemd yaml - Parser error
    
    Merges: #1384