Improve user-friendly error output for Fluent

[m-xim]

FTL file:

hello = Hello, <b>{ $firstname } { $lastname }</b>!

Before:

fluent.runtime.errors.FluentReferenceError: Unknown external: firstname

After:

aiogram_i18n.exceptions.FluentMessageError: 
2 errors for key 'hello':
  Unknown external: firstname (type=FluentReferenceError)
  Unsupported external type: lastname, <class 'NoneType'> (type=TypeError)

[m-xim]

@andrew000