Improve error message when a rule fails to generate a target

[rgrinberg]

See this issue for an example. In the case where a rule fails to generate a target we get the fairly cryptic error:

Error: Rule failed to generate the following targets:
- src/unix/dlllwt_unix_stubs.so

It would be nice to improve this message by including the Action that tried to produce this target and possibly a location (if it exists). Right now, we have to dig through the build logs.

@diml I'm willing to improve this if you think this is a good idea. I haven't looked at the code yet, but it seems like this would be fairly simply. Any thing I need to keep in mind?

I'm for it. I agree that it should be straightforward

[rgrinberg]

This was fixed a while ago.