Better support for exceptions and mixed compilers

I added a plumed_cmd_nothrow function to the interface, that is used from
C++. This allows better support for mixed compilers (e.g. MD code and PLUMED
compiled with different compilers or linked against different std libraries).

Notice that I tried in the past to achieve the same using cmd("setErrorHandler"),
but this does not work correctly with some combination of compilers.
In addition, it implies throwing from a extern "C" function, which is not standard
and might lead to portability issues.

The plumed_cmd_nothrow implementation catches any exception raised within plumed,
converts it to an error code, and rethrows it within the MD code. This approach
should be more robust.