Update error message to use new lexicon

[cpswan]

Is your feature request related to a problem? Please describe.

The present error message for AT0007 in packages/at_commons/lib/src/exception/error_message.dart is Secondary Server not found.

Describe the solution you'd like

Use atServer not found. instead

Describe alternatives you've considered

We might have to leave this be (or leave until we're doing other breaking changes).

Additional context

Could this break anything? Do we expect clients to look for the full error string or just the error code?

[cpswan]

@murali-shris I just realised we need to get somebody assigned to do the work on this, as I don't expect @CynthiaPeter to do that.

[cpswan]

Adding @gkc and bumping to PR57

[gkc]

Also need to update at_lookup_impl.dart in at_libraries

  static String getNotFoundExceptionMessage(String atSign) {
    return 'Unable to find secondary address for atSign:$atSign';
  }

and various other places https://github.com/search?q=org%3Aatsign-foundation+%22Secondary+server+not+found%22&type=code