More consistent and informative error messages, clearer backtrace

I've taken out commas and function names from error text.  I reduced the red in the backtrace to the function names only.  I've added more details to setindex! to inform the user about the dimensions of the arguments when there is an error.  

Something similar should probably be done for BoundsError(), like give it as msg attribute that gets printed.

Also, it would be nice to have DimensionMismatch from the linalg/ code available at the top level. I see a lot of errors that are essentially dimension mismatches.