pg_upgrade: fix CopyFile() on Windows to fail on file existence

Also fix getErrorText() to return the right error string on failure.
This behavior now matches that of other operating systems.

Report by Noah Misch

Backpatch through 9.1

src/bin/pg_upgrade/function.c

@@ -214,7 +214,7 @@ check_loadable_libraries(void)
 
 			if (script == NULL && (script = fopen_priv(output_path, "w")) == NULL)
 				pg_fatal("Could not open file \"%s\": %s\n",
-						 output_path, getErrorText(errno));
+						 output_path, getErrorText());
 			fprintf(script, "Could not load library \"%s\"\n%s\n",
 					lib,
 					PQerrorMessage(conn));