Add context to error messages.

New output:

Error:
    Problem with module re-exports:
      - The module 'Asdf'
        is not exported by any suitable package.
        It occurs in neither the 'exposed-modules' of this package,
        nor any of its 'build-depends' dependencies.
    In the stanza 'library'
    In the inplace package 'Reexport2-1.0'

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