Improve gendef.pl diagnostic on failure to open sym file

There have been numerous buildfarm failures but the diagnostic is
currently silent about the reason for failure to open the file. Let's
see if we can get to the bottom of it.

Backpatch to all live branches.

src/tools/msvc/gendef.pl

@@ -32,7 +32,7 @@ sub dumpsyms
 sub extract_syms
 {
 	my ($symfile, $def) = @_;
-	open(my $f, '<', $symfile) || die "Could not open $symfile for $_\n";
+	open(my $f, '<', $symfile) || die "Could not open $symfile for $_: $!\n";
 	while (<$f>)
 	{