* src/function.c (func_shell_base): Use error() instead of recreating
the error output.
* src/job.c (exec_command): Show more standard error messages.
* src/load.c (unload_file): Fix whitespace in the error message.
* tests/scripts/features/errors: Add tests for starting non-
existent commands and new error message formats.
* tests/scripts/features/output-sync: New error message formats.
* tests/scripts/functions/shell: Ditto.