Elixirc: fail when one of the provided file pattern is missing

The previous logic didn't allow for a simple fix (patterns were converted all at once and then everything was concatened, thus squashing all the empty lists == missing files).

In this PR, I am proposing to solve the issue #1923 by going through the list of patterns and collect all the patterns that returned no files. If there was at least one, then the process_command for :compile will return an error with a similar error message as before, except that now it's a bit more descriptive (only the missing patterns are mentioned). 

It wouldn't be very difficult to change the code to proceed with the compilation despite the missing files and just issue a warning, but it's probably more sane to just fail right away.