Can this be used to customize Drupal messages?

I'm trying to find a simple way to customize standard Drupal warnings and error messages - like "The URL of your homepage is not valid. Remember that it must be fully qualified, i.e. of the form http://example.com/directory."

I don't know if these are passed via t() and have no idea to figure that out. I tried using this module (which I use for other, simpler things) for the above statement (including just partial strings to void HTML glitches), but didn't work, so assume they must not be going thru t, right?

If this module won't work, do you know of something that will? I tried locale/language as is suggested in various threads here and elsewhere and am not seeing how to make that work. Search function does not see these statements, but existing instructional info is dated, so perhaps not doing it correctly.

Any suggestions? Thanks for your help.