Restore backward-compatibility layer for errors

Faraday has been maintaining a backward-compatibility layer for errors
in the `Faraday::Error::` "namespace". These were removed with v0.16.0,
which caused issues for dependents that use semantic versioning
constraints.

It would be best if these were deprecated with a warning message and
then dropped in v1.0 in order to make sure transitive dependencies do
not break.

See octokit/octokit.rb#1154

lib/faraday/error.rb

@@ -97,4 +97,9 @@ class ParsingError < Error
   # @see Faraday::Request::Retry
   class RetriableResponse < Error
   end
+
+  %i[ClientError ConnectionFailed ResourceNotFound
+     ParsingError TimeoutError SSLError RetriableResponse].each do |const|
+    Error.const_set(const, Faraday.const_get(const))
+  end
 end

spec/faraday/error_spec.rb

@@ -41,5 +41,11 @@
       it { expect(subject.message).to eq('["error1", "error2"]') }
       it { expect(subject.inspect).to eq('#<Faraday::ClientError #<Faraday::ClientError: ["error1", "error2"]>>') }
     end
+
+    context 'maintains backward-compatibility until 1.0' do
+      it 'does not raise an error for nested error classes' do
+        expect { Faraday::Error::ClientError }.not_to raise_error
+      end
+    end
   end
 end