git-gui: return early when patch fails to apply

In the procedure apply_or_revert_range_or_line, if the patch does not
apply successfully, a dialog is shown, but execution proceeds after
that. Instead, return early on error so the parts that come after this
don't work on top of an error state.

Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>

lib/diff.tcl

@@ -848,6 +848,8 @@ proc apply_or_revert_range_or_line {x y revert} {
 		puts -nonewline $p $wholepatch
 		close $p} err]} {
 		error_popup "$failed_msg\n\n$err"
+		unlock_index
+		return
 	}
 
 	unlock_index