linux-user: If loading fails, print error as string, not number

If the attempt to load the guest executable fails, print the
error message as a string, not a number. This requires us to
fix a couple of places in loader_exec() where we were returning
-1 instead of a valid negative errno.

The change allows us to drop the "Unknown binary format" message
because the strerror-enhanced message is now a more self-explanatory
"Error while loading $guest-binary: Exec format error".

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

linux-user/main.c

@@ -3569,7 +3569,7 @@ int main(int argc, char **argv, char **envp)
     ret = loader_exec(filename, target_argv, target_environ, regs,
         info, &bprm);
     if (ret != 0) {
-        printf("Error %d while loading %s\n", ret, filename);
+        printf("Error while loading %s: %s\n", filename, strerror(-ret));
         _exit(1);
     }