External Kotlin errors don't work

[mhammond]

#2390 added better support for Swift and Python, but making Kotlin work was tricky enough to split out into its own issue.

The problem is, unsurprisingly, RustBuffers and how we lift the errors. We generate a UniffiRustCallStatusErrorHandler for each module which is used to lift errors in that module, but UniffiRustCallStatusErrorHandler has a reference to this module's RustBuffer. When an external error is involved, UniffiRustCallStatusErrorHandler fails to work because the RustBuffer is the one from the external module.

So somehow we need to rejig the generic code here such that a reference to the remote RustBuffer can be specified.