Assert on invalid command line argument is not user friendly.

[lkarsten]

When implementing the automake test driver, an incorrect path error lead to the following output:

./tests/test02-simple-request: line 5: common.sh: No such file or directory
Assert error in front_arg_add(), configuration.c line 565:
  Condition((fa->port) != 0) not true.
./tests/test02-simple-request: line 8:  8570 Aborted                 (core dumped) hitch $HITCH_ARGS --backend=[hitch-tls.org]:80 "--frontend=[${LISTENADDR}]:$LISTENPORT" certs/site1.example.com
FAIL: tests/test02-simple-request

Asserting when the user calls hitch with bad arguments isn't very user friendly and should be improved.

This was on 9265839.

Not sure which part is producing the assert. Since common.sh isn't in place, the working directory is not src/tests/ so most likely certs/site1.example.com doesn't exist.