Fix webfinger retries (#4275)

* Do not raise unretryable exceptions in ResolveRemoteAccountService

* Removed fatal exceptions from ResolveRemoteAccountService

Exceptions that cannot be retried should not be raised. New exception
class for those that can be retried (Mastodon::UnexpectedResponseError)

spec/services/resolve_remote_account_service_spec.rb

@@ -22,11 +22,11 @@ RSpec.describe ResolveRemoteAccountService do
   end
 
   it 'raises error if no such user can be resolved via webfinger' do
-    expect { subject.call('catsrgr8@quitter.no') }.to raise_error Goldfinger::Error
+    expect(subject.call('catsrgr8@quitter.no')).to be_nil
   end
 
   it 'raises error if the domain does not have webfinger' do
-    expect { subject.call('catsrgr8@example.com') }.to raise_error Goldfinger::Error
+    expect(subject.call('catsrgr8@example.com')).to be_nil
   end
 
   it 'returns an already existing remote account' do
@@ -58,7 +58,7 @@ RSpec.describe ResolveRemoteAccountService do
   end
 
   it 'prevents hijacking inexisting accounts' do
-    expect { subject.call('hacker2@redirected.com') }.to raise_error Goldfinger::Error
+    expect(subject.call('hacker2@redirected.com')).to be_nil
   end
 
   it 'returns a new remote account' do