Catching Regulus errors

Peter Ljunglöf wondered whether error reporting in Regulus could be improved, and had a couple of suggestions. I've implemented and checked in the following improvements:

1. All error messages should now be printed to stderr.
   
   
2. When processing fails during execution of the Regulus command , a line of the form
   
   Error processing command:
   
   should be printed. This was not previously the case.
   
   
3. There is a new top-level predicate
   
   regulus_batch_storing_errors(+ConfigFile, +Commands, -ErrorString)
   
   which is like regulus_batch/2, except that it instantiates ErrorString with a string containing all the errors printed out during execution of Commands.

I expect there will be some glitches (I had to change a lot of lines of code), so please let me know if thing don't work as intended.