#1 Make <prefix_error> optional

[Daniel Prevost]

No time to work on this right now so I'm writing it down to make sure it does not get lost (or if someone else wants to jump in!)...

The idea behind <prefix_error> is simply to make it convenient to change the error codes (from PROJ1_ERROR1 to PROJ2_ERROR1) for things like embedding a library into another library or a fork or...).

But, in many cases it is probably more convenient to just define the error names with the full name and to forget about this <prefix_error> tag.

On a related note, if the tag is empty, it should be handled as if the tag was not present (the current code adds a "_" at the beginning - this might be considered a bug...).