Return a value from Install.pm's lcopy function

Commit 3a7cc727 was a little over eager about adding an explicit return
to this function, whose value is checked in most call sites. This change
reverses that and returns the expected value explicitly. It also adds a
check to the one call site lacking one.

src/tools/msvc/Install.pm

@@ -37,10 +37,10 @@ sub lcopy
 		unlink $target || confess "Could not delete $target\n";
 	}
 
-	copy($src, $target)
+	(my $retval = copy($src, $target))
 	  || confess "Could not copy $src to $target\n";
 
-	return;
+	return $retval;
 }
 
 sub Install
@@ -200,7 +200,7 @@ sub CopyFiles
 		print ".";
 		$f = $basedir . $f;
 		die "No file $f\n" if (!-f $f);
-		lcopy($f, $target . basename($f));
+		lcopy($f, $target . basename($f)) || croak "Could not copy $f: $!\n";
 	}
 	print "\n";
 	return;