perf: Improved endpoint handling

Recent improvements in webhook endpoint message handling, in conjunction with another round of developer-tool-empowered webhook simulation with @mattlav, has shaken loose a few inconsistencies or insufficiencies in how each webhook handler has been treating incoming messages.

This MR addresses these in the following ways:

• One-time donation failures via Paypal now explicitly handled
• Recurring payment successes and failures via Stripe no longer register additional, erroneous one-time payment successes or failures (respectively)
• Linter complaints about regarding message being undefined in theoretical edge cases addressed via refactoring of variable scope and definition