Revert "Server::step throw is never catched. Replace it with an errorstream + assert"

This reverts commit 5f8e48c63b.

src/server.cpp

@@ -479,13 +479,10 @@ void Server::step(float dtime)
 		JMutexAutoLock lock(m_step_dtime_mutex);
 		m_step_dtime += dtime;
 	}
-	// Assert if fatal error occurred in thread
+	// Throw if fatal error occurred in thread
 	std::string async_err = m_async_fatal_error.get();
 	if(async_err != ""){
-		errorstream << "UNRECOVERABLE error occurred. Stopping server. "
-				<< "Please fix the following error:" << std::endl
-				<< async_err << std::endl;
-		assert(false);
+		throw ServerError(async_err);
 	}
 }