Fix handling of missing nested include-files.

Generated an exception instead of the intended error message.

e.g.:
```
# suite.rc
%include foo.rcf
```
```
# foo.rc
%include missing.rc
```
Validation should say:
```
ParseError: File not found: /path/to/missing.rc
   via /path/to/foo.rc
   via /path/to/suite.rc
```
not:
```
cannot concatenate 'str' and 'list' objects
```

@matthewrmshin - please review.