Improve exception handling in Flodesk::Client

Currently we've got the following code in Client#request:

https://github.com/codebar/planner/blob/45fd9d14f91bbbc80c13412983b51d75ff285c57/lib/flodesk.rb#L128-L133

Make code nil-safe

This can result in a NoMethodError: undefined method '[]' for nil if there is an issue connecting to Flodesk and the response doesn't actually contain a message_body.

This could happen in production, but also can be a source of flaky tests as happened recently.

Proposing to make this nil-safe by using e.response_body&.['message']

Raise rather than return the exception

Currently this code is actually returning the FlodeskError rather than raising it.

Proposing to raise the exception instead, but note there's one place where we currently check the return type from the method that will need updating to reflect this change:

https://github.com/codebar/planner/blob/45fd9d14f91bbbc80c13412983b51d75ff285c57/lib/flodesk.rb#L70-L74

Mock out the client in tests

As a separate task we can take a look at mocking Flodesk::Client in the specs. → Fixed on https://github.com/codebar/planner/pull/2402

Start in lib/flodesk.rb at Client#request and inspect the return-type check around lines 70–74. Verify the connection-error path when response_body is absent, update the caller behavior, and run the existing Flodesk-related specs to confirm errors are raised and the nil case no longer causes a NoMethodError.