Better error message if include_lib doesn't exist

[tsloughter]

Currently it just crashes:

===> Uncaught error in rebar_core. Run with DEBUG=1 to see stacktrace
===> Uncaught error: function_clause
===> Stack trace to the error location: [{filename,join,
                                                 [[]],
                                                 [{file,"filename.erl"},
                                                  {line,393}]},
                                                {rebar_erlc_compiler,
                                                 warn_and_find_path,2,
                                                 [{file,
                                                   "/home/tristan/Devel/rebar3/_build/default/lib/rebar/src/rebar_erlc_compiler.erl"},
                                                  {line,691}]},