fix(api): failover error bugs

[samlaf]

Why are these changes needed?

2 big bugs in my ErrorFailover implementation:

1. I misunderstood how errors.Is works, so could only compare if the underlying wrapped error was exactly equal, which is not useful (and made the logic in disperser/eigenda-clients incorrect)
2. err.Error() would panic when ErrorFailover{} was created with a nil wrapped error

Added tests to make sure the above are fixed, forever.

Checks

• I've made sure the lint is passing in this PR.
• I've made sure the tests are passing. Note that there might be a few flaky tests, in that case, please comment that they are not relevant.
• I've checked the new test coverage and the coverage percentage didn't drop.
• Testing Strategy
  • Unit tests
  • Integration tests
  • This PR is not tested :(