Improve error output when package fails to build.

New error message is:

cabal: Failed to build p-1.0 (which is required by exe:e4 from p-1.0, exe:e3
from p-1.0 and others).

Fixes #4081.

TODO: Use these utility functions in other appropriate places.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>