🥅 Add new InvalidResponseError exception class

The documentation for `InvalidResponseError` is:
> Error raised when the server sends an invalid response.
>
> This is different from UnknownResponseError: the response has been
> rejected.  Although it may be parsable, the server is forbidden from
> sending it in the current context.  The client should automatically
> disconnect, abruptly (without logout).
>
> Note that InvalidResponseError does not inherit from ResponseError: it
> can be raised before the response is fully parsed.  A related
> ResponseParseError or ResponseError may be the #cause.

The rdoc for `UnknownResponseError` is updated to:
> Error raised upon an unknown response from the server.
>
> This is different from InvalidResponseError: the response may be a
> valid extension response and the server may be allowed to send it in
> this context, but Net::IMAP either does not know how to parse it or
> how to handle it.  This could result from enabling unknown or
> unhandled extensions.  The connection may still be usable,
> but—depending on context—it may be prudent to disconnect.

All code that previously raised `UnknownResponseError` now raises
`InvalidResponseError`.  However, the class is kept both for backward
compatibility and because it might be reused in the future for similar
scenarios: e.g: when a tag doesn't match any outstanding response tags.

lib/net/imap.rb

@@ -2519,7 +2519,8 @@ def get_tagged_response(tag, cmd, timeout = nil)
       when /\A(?:BAD)\z/ni
         raise BadResponseError, resp
       else
-        raise UnknownResponseError, resp
+        disconnect
+        raise InvalidResponseError, "invalid tagged resp: %p" % [resp.raw.chomp]
       end
     end
 

lib/net/imap/errors.rb

@@ -47,7 +47,27 @@ class BadResponseError < ResponseError
     class ByeResponseError < ResponseError
     end
 
+    # Error raised when the server sends an invalid response.
+    #
+    # This is different from UnknownResponseError: the response has been
+    # rejected.  Although it may be parsable, the server is forbidden from
+    # sending it in the current context.  The client should automatically
+    # disconnect, abruptly (without logout).
+    #
+    # Note that InvalidResponseError does not inherit from ResponseError: it
+    # can be raised before the response is fully parsed.  A related
+    # ResponseParseError or ResponseError may be the #cause.
+    class InvalidResponseError < Error
+    end
+
     # Error raised upon an unknown response from the server.
+    #
+    # This is different from InvalidResponseError: the response may be a
+    # valid extension response and the server may be allowed to send it in
+    # this context, but Net::IMAP either does not know how to parse it or
+    # how to handle it.  This could result from enabling unknown or
+    # unhandled extensions.  The connection may still be usable,
+    # but—depending on context—it may be prudent to disconnect.
     class UnknownResponseError < ResponseError
     end
 

test/net/imap/test_imap.rb

@@ -147,9 +147,10 @@ def test_starttls_stripping
       imap = nil
       starttls_stripping_test do |port|
         imap = Net::IMAP.new("localhost", :port => port)
-        assert_raise(Net::IMAP::UnknownResponseError) do
+        assert_raise(Net::IMAP::InvalidResponseError) do
           imap.starttls(:ca_file => CA_FILE)
         end
+        assert imap.disconnected?
         imap
       end
       assert_equal false, imap.tls_verified?