[web] handle case where user needs to log in from native first

Summary:
an unknown error here is not very helpful, so we should explain to the user why login failed and instruct them to try from native

would normally add @ashoat since this diff introduces new copy. happy to incorporate any feedback when he returns

Depends on D12544

Test Plan:
tested later in stack by registering a password and wallet user directly with my keyserver, which in turn called identity and added the address and username to the reserved usernames table

then, i flipped usingCSAT to true and tried logging in from web, which failed and displayed a message saying to log in from a mobile device

Reviewers: inka, ginsu

Reviewed By: inka

Subscribers: tomek, ashoat

Differential Revision: https://phab.comm.dev/D12545