Handle Reva REST API call status codes correctly.

[MahdiBaghbani]

Currently we don't check the status codes of calling the Reva APIs, example:

nc-sciencemesh/lib/RevaHttpClient.php

Lines 114 to 120 in e22483b

	public function generateTokenFromReva(string $userId, string $recipient)
	{
	// TODO: @Mahdi: handle failures in this request.
	$tokenFromReva = $this->revaGet('sciencemesh/generate-invite', $userId, array('recipient' => $recipient));
	error_log('Got token from reva!' . $tokenFromReva);
	return json_decode($tokenFromReva, true);
	}

Proper handling and throwing exceptions / or informing user about problem is required.