help: warn if specified 'man.viewer' is unsupported, instead of erroring out

When a document viewer that is unknown to the current version of git is
specified in the .git/config file, instead of erroring out the process
entirely, just issue a warning.  It might be that the user usually is
using a newer git that supports it (and the configuration is written for
that version) but is temporarily using an older git that does not know the
viewer.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

help.c

@@ -139,7 +139,7 @@ static int add_man_viewer(const char *value)
 	else if (!strcasecmp(value, "konqueror"))
 		do_add_man_viewer(exec_man_konqueror);
 	else
-		return error("'%s': unsupported man viewer.", value);
+		warning("'%s': unsupported man viewer.", value);
 
 	return 0;
 }