Print any error in exec driver

This should not happen, so it's fine to print
a stack trace if something bad happens
Improve regex handling

We actually need to redo the regex handling all
together: we need to parse and understand
erlang (pcre) regex, and then generate an 
equivalent java one.
Improve efile posix error codes

For some errors, we were reporting the wrong posix
errors.
Don't include erlang:apply/3 in stack traces

Apparently erlang doesn't do this, and Elixir
check for it.