Update error messages to be more descriptive.

@pisarna @tbold @ponyisi

I've updated a few of the error messages.

1. In AthMonitorAlgorithm, the error getGroup function told you which group name was missing, but now it also tells you from which algorithm (in case there are multiple instances of the same algorithm in a single job.)

2. In HistogramDef, warns the user that the parsing error may have been caused by a bad use of delimiters. One user was using commas in the title, and had trouble since he presumably thought that the error was casting float to int, or something like that. If there's a better way to do this one, I can certainly try it.

Chuck