Improve unw_get_proc_name error check

The length param of unw_get_proc_name already takes care of the
terminator, so remove the subtraction.

The manual describes that the error code is given by the negative, so
fix the condition checking against UNW_ENOMEM (verified by shrinking the
buffer).

Finally, the error message confused me (as a user) as it seems to
suggest something about the process name (rather than procedure).
Mention the more applicable "frame" instead (longer alternative:
"procedure name of frame").