[Validation] error messages

[tenmajkl]

This PR adds error messages to validation, meaning that now, when validation error ocures, instead of just saying "Bad data" you get better errors. So if we pass request

foo=bar

into this validation

$r->validate([
    'foo' => 'numeric',
], template('foo'));

with this template

{ ifError }
    <div class="error">
        { error }
    </div>
{ /ifError }

response would be div with class error and message Value foo must be numeric.

All the error messages are from translations.

Resolves #112