compare_builds returns message instead of variable for an unfinished build

[jscook2345]

compare_builds is expected to return a dict, however when one of the builds is not finished, you get the following:

['Cannot report regressions/fixes on a non-finished builds']

I'm not sure this is correct behavior. Perhaps it should return something else (none?) and squad-client can throw an exception letting me know that it cannot work on unfinished builds

[amrohassaan]

@jscook2345 but how to show the error to those looking at the UI? Maybe we can add something to client to test if build.finished==true before calling executing the actual compare, then throwing the error if not.