Make failure to open psql's --log-file fatal.

Commit 344cdff2c made failure to open the target of --output fatal.
For consistency, the --log-file switch should behave similarly.
Like the previous commit, back-patch to 9.5 but no further.

Daniel Verite

src/bin/psql/startup.c

@@ -274,8 +274,11 @@ main(int argc, char *argv[])
 	{
 		pset.logfile = fopen(options.logfilename, "a");
 		if (!pset.logfile)
+		{
 			fprintf(stderr, _("%s: could not open log file \"%s\": %s\n"),
 					pset.progname, options.logfilename, strerror(errno));
+			exit(EXIT_FAILURE);
+		}
 	}
 
 	/*