Wrong error message when running "make test" when cx executable does not exist

[SkycoinSynth]

"make test" gives wrong error message when cx executable does not exist or is not in $PATH

"#112 | FAILED | na | 'cx issue-282b.cx' | os.Run exited with code os.RUN_START_FAILED (3) | Panic when inserting a new line in a string literal"

Reports "os.RUN_START_FAILED" instead of cx executable does not exist. Need to add more error messages.