Fix cut-and-paste errors in nonfatal() implementations.

Fix cut-and-paste errors in nonfatal() implementations.

Unix GUI programs should not say 'Fatal Error' in the message box
title, and Plink should not destroy its logging context as a side
effect of printing a non-fatal error. Both appear to have been due to
inattentive cut and paste from the pre-existing fatal error functions.

[originally from svn r10044]