Show any error messages from dlopen

[tkelman]

Mostly a reminder to myself to figure out how to do this, if no one else gets to it before I can.

Dependency problems (like libgfortran not having a certain symbol, in my case) are virtually impossible to debug from just LoadError: Provider BinDeps.BuildProcess failed to satisfy dependency libfoo. If the problem is coming from a dlopen error, would be best to show the full error instead of relying on manually trying dlopen (and even knowing to try that as a debugging option) to figure out what's wrong.

[staticfloat]

It would be nice to auto-run BinDeps.debug() when something fails. That's what we want to do 50% of the time, and the other 50% we just want to see the errors from dlopen().