Merge rust-bitcoin#3801: units: Make amount::error module private

6e78c05 api: Run just check-api (Tobin C. Harding)
760f071 units: Make amount::error module private (Tobin C. Harding)

Pull request description:

  The `untis::error` module is just a code organisation thing it should never have been public. We already re-export all the error types and this is verified by the `units/tests/api.rs` test file.

  Make the module private and remove it from the paths in the `api` test.

ACKs for top commit:
  apoelstra:
    ACK 6e78c05; successfully ran local tests; man, these API tests are awesome

Tree-SHA512: eb1fd6d0a9c49b49c275782e6525e904ff72536260a1fba965140fab5a62f53db13d06a294e5efbc54d500a03d9fc7322b16b8154e587a5f0ff4e7d5e2c0288e