Accept environment variables spelled with underscores

[kuhnmi]

Environment variables with dots are hard to handle in unix shells (e.g. bash). A quick google search for 'environment variables dots' shows that many people struggle with such variables, e.g.:

http://unix.stackexchange.com/questions/93532/exporting-a-variable-with-dot-in-it
http://askubuntu.com/questions/65638/how-to-set-env-with-a-dot
http://stackoverflow.com/questions/2821043/allowed-characters-in-linux-environment-variable-names

To make custom configurations easier on unix systems, this PR makes find-sec-bugs also accept environment variables spelled with underscores (_) instead of dots (.). E.g. findsecbugs_taint_customconfigfile instead of findsecbugs.taint.customconfigfile. Using environment variables (rather than system properties) for configuration is e.g. helpful when using find-sec-bugs with gradle.

Additionally, I've added a small change that fails early if some custom file/resource cannot be found. Otherwise it would just fail a bit later with an NPE not telling which file/resource could not be found.