avoid Arg.Bad on non-existent file
Before:
$ ./compiler/heptc.byte nosuchfile.ept
./compiler/heptc.byte: Unknown file: nosuchfile.ept.
Options are:
-v Set verbose mode
-version The version of the compiler
[...]
-help Display this list of options
--help Display this list of options
After:
$ ./compiler/heptc.byte nosuchfile.ept
Error: file nosuchfile.ept does not exist.