Improve error message when ACE fails to start

[goodmami]

Instantiating an AceProcessor with an old grammar image currently results in a stack trace with the error "Broken pipe" which hides the error from ACE. The delphin.interfaces.ace.AceProcess._open() method should check if the opened process terminated and raise a more appropriate error (this may require a new exception class in delphin.exceptions)

[mcmillanmajora]

I think I have a solution for this by checking if the Popen object's poll() value is not None, but I'm not sure what kind of file I would need write to replicate an old grammar image for testing.

[goodmami]

Good :) And that sounds about right. Checking the poll() value is how I detect crashes in the _result_lines() method of AceProcess. As for testing, look how I use monkeypatch (a pytest thing) to override stdin in tests/commands_test.py. You can use this pattern with the expected output from ACE (the "version mismatch..." line below). It would be nice if you could also override the return code (shown by the $? variable in bash), but I'm not sure offhand how to do that.

goodmami@tpy:~/grammars$ ace -g erg-1214-x86-64-0.9.27.dat 
version mismatch: this is ACE version 0.9.29, but this grammar image was compiled by ACE version 0.9.27
goodmami@tpy:~/grammars$ echo "$?"
255

Also, I'm not sure if you've looked at the v1.0.0 branch yet, but I've moved to have a fairly minimal delphin.exceptions with module-specific exceptions defined in the modules themselves (in case you created a custom exception for this ACE error). Let me know if you have questions about this.

[goodmami]

@mcmillanmajora could you submit a PR? It should be easier to figure out the remaining issues that way.

[goodmami]

Assigning to myself. I have a fix in my branch.