core: clean shutdown during boot in case of dependency start failures.

Instead of getting a nasty looking:
{"init terminating in do_boot",{{case_clause,{error,{"no such file or directory","xmerl.app"}}},[{zotonic,ensure_started,1},{zotonic,ensure_started,1},{zotonic,start,1},{init,start_it,1},{init,start_em,1}]}}

Give a:
10:14:41.213 [error] Zotonic failed to start application: no such file or directory: xmerl.app