Distinguish between exception instances and exceptions that are actually raised during reduction #112

We will introduce a ReductionException type which will wrap exceptions that are 
raised during reduction (i.e. reduceFully, uquote, text, raises, and raisesWith 
operators and extension methods). This will allow us to e.g. improve 
decompilation so that values of non-raised exceptions aren't printed with 
verbose stacktrace information. Note that this will be a slight breaking change 
in that the last expression returned from reduceAll will contain the 
ReductionException instead of the actual exception if one is thrown (use 
ReductionException.InnerException to get at the actual Exception).