Handle http 422 error code properly

[cztomczak]

There is a "http_error" callback in Mongoose that maybe we should implement to handle 422 error code:

phpdesktop/src/mongoose.h

Line 125 in 3a56b93

// Called when mongoose is about to send HTTP error to the client.

This is the default "send_http_error" implementation for handling http errors in Mongoose:

phpdesktop/src/mongoose.c

Line 953 in 3a56b93

static void send_http_error(struct mg_connection *conn, int status,

Reported on the Forum:
https://groups.google.com/d/topic/phpdesktop/RcB3q2uiwGc/discussion