This adds an additional loading and merging of options for EDoc using
the values from the top-level along with those specified in the
rebar.config of an umbrella application.

The app-specific config values are prepended to the global ones; this
can likely cause some problems with manual path handling, but is
unlikely to happen in practice and the rest seems to work fine based on
order

Fixes the issue in #2114

Instead of a hard crash, display an error message indicating which app
failed.

We can't report the exact failure; only EDoc does it to stdout itself
and we can't capture it.

Pre/Post hooks are run properly despite the failure, as per escript and
compile providers.