add better error handling for invalid client URLs

[khancode]

High Level Overview of Change

Add better error handling for invalid client URLs.

Context of Change

If you put in the wrong URL of a node for a client, the error that it returns is super opaque (it's a JSONDecodeError). We should return a better error/error message, so that it's clear that the user entered the wrong URL for the node.

This change now raises an XRPLRequestFailureException if the response can't be JSON decoded (JSONDecodeError).

#353

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Tests (You added tests for code that already exists, or your new feature included in this PR)
• Documentation Updates
• Release

Test Plan

Added unit tests.

[JST5000]

LGTM