Print error details if an error occurs during compilation

I'm currently having some severe issues trying to get flylatex to run behind an NGinx reverse proxy on my VPS.

When trying to compile, the application just crashes -- I tracked down the bug, it's because of (IMO) incorrect behaviour of *fs-extra*, see https://github.com/jprichardson/node-fs-extra/pull/38 .

However, even after I fixed it, I didn't find the flylatex error messages very helpful -- no detailed error message was console-logged, and the webinterface also printed only "An error occured before or during compilation", without any details.

This PR fixes this by console-logging errors and by printing the error details on the web interface.