improve error message generated for a missing easyconfig file

easyconfig filename being looked for is now included in error message:

```
Failed to find easyconfig file 'imkl-10.3.12.361.eb' when determining module name for: {'dummy': True, 'versionsuffix': '', 'version': '10.3.12.361', 'toolchain': {'version': 'dummy', 'name': 'dummy'}, 'name': 'imkl', 'full_mod_name': None, 'hidden': False, 'short_mod_name': None}
```

suggestion by @treydock