chore: provide informational error message when api key env var is not set

[anitarua]

addresses #77

Before:

Unhandled exception:
API key is an empty string

After:

Unhandled exception:
Environment variable MOMENTO_API_KEY is required and not set

[anitarua]

Converting to draft while I simplify the EnvMomentoTokenProvider class based on Pete's feedback: no longer a subclass of StringMomentoTokenProvider so that super does not get called before the env var is checked, and the named constructors were removed in favor of having just one constructor in each TokenProvider class that accepts optional arguments for overriding the endpoints