Don't save "warning: " prefix in lastwarn() message. * error.cc (vwarning): Save base_msg (without "warning: ") to Vlast_warning_message. * error.cc (error_1): Use std::string operators rather than strsave to strip "\n" from fmt string. Simpler syntax avoids having to delete [] memory created with new. * error.cc (warning_1): Strip trailing newline from fmt string as error_1 does. * fail.m: Fix input validation which allowed nonsensical second input with 3 args. Don't post-process warning or error messages from lastwarn or lasterr. Add BIST test for input validation.

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