Do not crash Winscope when loading viewers fails.

Screencast: https://screencast.googleplex.com/cast/NTQwMzA3NDk3NjgwODk2MHxhNzBlOGY0OS01NA

Improve user warnings/errors.

Bug: 349129695
Test: npm run test:unit:ci
Change-Id: I4fb95ae216f2bab1ca191a0931c0737692553a2a
9 files changed