Load caps return status code sometimes different than 500

[MDario123]

This pull request includes several changes to improve error handling and streamline code in the src/caps.rs file. The most important changes involve refactoring error handling to use a custom ScratchError type and updating function signatures accordingly.

Improvements to error handling:

• src/caps.rs: Changed the return type of load_caps_buf_from_url and load_caps functions to use ScratchError instead of String for error handling. [1] [2]

Codebase simplification:

• src/custom_error.rs: Added a new helper function ScratchError::new_internal to create ScratchError instances with a default INTERNAL_SERVER_ERROR status code.
• src/global_context.rs: Updated error handling in try_load_caps_quickly_if_not_present to use the status_code from the original error when creating a new ScratchError.

Dependency updates:

• src/caps.rs: Reorganized imports to improve readability and maintain consistency.

[alashchev17]

Error returned is still being inferred

[alashchev17]

Now it works, frontend already relies on this PR in release, we need to merge this