Fixes #2418: Enhance wiggle to generate its UserErrorConverstion trait with a function that returns Result<abi_err, String> (#2419)

* Enhance wiggle to generate its UserErrorConverstion trait with a function that returns
a Result<abi_err, String>.  This enhancement allows hostcall implementations using wiggle
to return an actionable error to the instance (the abi_err) or to terminate the instance
using the String as fatal error information.

* Enhance wiggle to generate its UserErrorConverstion trait with a function that returns
a Result<abi_err, String>.  This enhancement allows hostcall implementations using wiggle
to return an actionable error to the instance (the abi_err) or to terminate the instance
using the String as fatal error information.

* Enhance the wiggle/wasmtime integration to leverage new work in ab7e9c6.  Hostcall
implementations generated by wiggle now return an Result<abi_error, Trap>.  As a
result, hostcalls experiencing fatal errors may trap, thereby terminating the
wasmtime instance.  This enhancement has been performed for both wasi snapshot1
and wasi snapshot0.

* Update wasi-nn crate to reflect enhancement in issue #2418.

* Update wiggle test-helpers for wiggle enhancement made in issue #2418.

* Address PR feedback; omit verbose return statement.

* Address PR feedback; manually format within a proc macro.

* Address PR feedback; manually format proc macro.

* Restore return statements to wasi.rs.

* Restore return statements in funcs.rs.

* Address PR feedback; omit TODO and fix formatting.

* Ok-wrap error type in assert statement.

crates/wiggle/tests/pointers.rs

@@ -157,7 +157,7 @@ impl PointersAndEnumsExercise {
             self.input3_loc.ptr as i32,
             self.input4_ptr_loc.ptr as i32,
         );
-        assert_eq!(e, types::Errno::Ok.into(), "errno");
+        assert_eq!(e, Ok(types::Errno::Ok.into()), "errno");
 
         // Implementation of pointers_and_enums writes input3 to the input2_loc:
         let written_to_input2_loc: i32 = host_memory