Define a clear service level agreement. List all the errors they have pointed out so far, do not seem like you are shirking responsibility, and in this (likely spreadsheet format) list the corrective action needed.

Meet them, too. For a coffee, tea, beer, whatever. I say this because by accepting and taking responsibility for what you have created, it is likely the developers of other components have included no support / feedback channel. This is both a risk and opportunity: the opportunity is an on-going chance to bill for services or future development; the risk is being seen as blaming others for your mistakes.