Simplify Instance of XYZ Error Message

[thibistaken]

A user created an invoice for 1500 SATs with 950 SATs in Liquid and 200k SATs in Bitcoin. The wallet attempted a BTC-to-LN swap but failed with an "Instance of XYZ" error due to insufficient balance for the swap limits. This vague error message should be replaced with a clear "insufficient balance" message.

Fix:

• We should never see Instance of XYZ error
• The error here is insufficient balance