git-send-email: ask what to do with an invalid email address

We used to warn about invalid emails and just drop them. Such warnings
can be unnoticed by user or noticed after sending email when we are not
giving the "final sanity check [Y/n]?"

Now we quit by default.

Signed-off-by: Krzysztof Mazur <krzysiek@podlesie.net>
Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -852,8 +852,16 @@ sub extract_valid_address_or_die {
 sub validate_address {
 	my $address = shift;
 	if (!extract_valid_address($address)) {
-		print STDERR "W: unable to extract a valid address from: $address\n";
-		return undef;
+		print STDERR "error: unable to extract a valid address from: $address\n";
+		$_ = ask("What to do with this address? ([q]uit|[d]rop): ",
+			valid_re => qr/^(?:quit|q|drop|d)/i,
+			default => 'q');
+		if (/^d/i) {
+			return undef;
+		} elsif (/^q/i) {
+			cleanup_compose_files();
+			exit(0);
+		}
 	}
 	return $address;
 }