Have "stack ghci" ignore build errors

[mgsloan]

While trying out the repro for #1067, it failed to find the autogenerated Paths_ghcid. This was because of two things:

1. The resolution of stack ghci should be usable with minimal dependencies #817 was to not do a build if there are no explicit targets. I don't think this is the right solution to the problem, and it makes the behavior of stack ghci inconsistent.

2. The resolution of stack ide start fails to start if the project has compile errors #810 was to only build the dependencies. I don't think this is the right solution to the problem, because if the project has never been built then autogenerated modules won't exist.

I think the solution is to remove the changes from #817 and #810 and instead adopt the following resolution:

Ignore build errors which occur during the implicit build which comes before running ghci. This will consist of catching ExcecutionFailure exceptions, thrown from here, and warn the user that the errors were ignored.

[borsboom]

#1228 includes what I think you're proposing. Although that one catches any synchronous exception, not just ExecutationFailure.

[mgsloan]

Yep, that nearly covers it! I've turned point (1) into a comment on the PR.