Error 500 when saving PM with invalid "To" field

Scenario 1

1. Go to private messages
2. Click "Compose"
3. Fill field "To" with login that not registered
4. Fill field "Subject" with some valid symbols
5. Click "Save"

Actual result: Error 500 appears
Expected result: Message should be saved with empty username

Scenario 2

1. Go to private messages
2. Click "Compose"
3. Fill field "To" with login that not registered
4. Leave "Subject" and "Message content" fields empty
5. Click "Save"

Actual result: "User not found" error is shown and drafts page without new draft is open at once
Expected result: "User not found" error is shown and message composing page isn't closed