Improve error message semantics

[apexDev37]

Description

This PR introduces a minor patch to enhance error message semantics.

# Current impl.

def initialize(username, api_key):
    if username is None or api_key is None:
        raise RuntimeError("Invalid username and/or api_key")

Not only is the above message misleading, but it's also logically incorrect.
"Invalid" infers some form of malformed input or failed validation, which requires existing data. Here, we only check if both username and api_key exist. This means we raise on the fact of non-existent data, but proceed to tell the user the data is invalid.

Patches

• Keep RuntimeError but update error message
• Add reference to official API docs for actionable insight to resolve the error