Import Wallet suggestions / issues

[riongull]

1. The page should probably default to a 24-word phrase if that's what the Wallet > Create Wallet creates by default. Personally I would prefer 12-word defaults for both pages.

2. There is no user feedback about faulty phrases. The app does check for valid phrases, but it doesn't give the user feedback about errors. The page should at least tell the user:

• When non-SLIP39 words are used.
• If the checksum fails.