Add new method to return the full error message string

[peverwhee]

Currently, hist_log_messages has a procedure to print the messages to a log unit, but has no way to return the string to the calling function.

It'd be nice to have that functionality so that we could send the message(s) to an endrun call, for example.

This was noted by @nusbaume during code review for ESCOMP/CAM-SIMA#487