fix: improve error messages for invalid API requests

[vivianamarquez]

When an invalid ID is passed to a command (e.g. prolific study view badid), the CLI dumps the raw API response body as the error message, which can be hundreds of lines of unformatted text.

Expected behavior: a short, human-readable message like "resource not found" or "request failed with status 404".

This affects all commands since the error handling is in the shared Execute method in client/client.go — any 4xx response with an unrecognized error format gets its full body printed.

[benmatselby]

@vivianamarquez Do you have PROLIFIC_DEBUG=1 by any chance?

I get this, which still probably isn't great.

❯ prolific study view badid
Error: error: unable to fulfil request /api/v1/studies/badid: request failed: The resource requested was not found or you do not have permissions.
Usage:
  prolific study view [flags]

Examples:

To get details about a study
$ prolific study view 64395e9c2332b8a59a65d51e

Flags:
  -h, --help   help for view
  -W, --web    Open the study in the web application

Global Flags:
      --config string   config file (default is $HOME/.config/prolific-oss/prolific.yaml)

error: unable to fulfil request /api/v1/studies/badid: request failed: The resource requested was not found or you do not have permissions.