Improve error handling for .wasm importing

[SpaceManiac]

Follow-up to #236.

Main fixes:

1. Skip functions which accept/return untranslateable types, rather than panicking.
2. Report a TypeError E0060 if importing a .wasm file fails, rather than panicking.
3. Restore the error on importing a file with an unknown extension, rather than silently ignoring it.

Also:

• Merge common parts of TypeError and TypeWarning into a generic StarError<T>.
• Add ImportWasmError type instead of using ad-hoc miette!() Reports.
• Remove unused top-level Error type.
• Combine ModuleGraphError::EntryIo and ImportIo, since keeping them separate complicates the implementation a bit.

Future work:

• For functions which were skipped, error if they're imported, rather than acting as though they're not found.
• Support importing functions which use record/variant types.
• Support importing record/variant types themselves.
• Support importing functions within interfaces, not just those bare on the world.