Merge bitcoindevkit/bdk#2137: Replace `std::error::Error` with `core::error::Error`

Merge bitcoindevkit/bdk#2137: Replace `std::error::Error` with `core::error::Error`

2a92fd365ea7f45e56b3027989b5cdf79ef4773a chore: use `core::error::Error` (Luis Schwab)

Pull request description:

  Closes #2132

  The `Error` trait is stable as of 1.81.0, so import it from `core` instead.

ACKs for top commit:
  evanlinjin:
    ACK 2a92fd365ea7f45e56b3027989b5cdf79ef4773a

Tree-SHA512: ce843843a932815b0991b4f5d064471876f103a4d2d8f12ac47eecfd1263ff79a833e937e81f1a98303f9002115dc792b6f4837d8752f49c6ab080762d510ee4