Further improve diagnostics for signatures with no SQL form (pgcentralfoundation#2215)

While we use `do_not_recommend` on the various implementations of
`FunctionMetadata`, it still appears as "this bound was not fulfilled".
Provide a further hint about why.

Author: workingjubilee

pgrx-sql-entity-graph/src/metadata/function_metadata.rs

@@ -33,6 +33,10 @@ assert_eq!(
 );
 ```
  */
+#[diagnostic::on_unimplemented(
+    message = "cannot generate SQL schema for this function",
+    label = "some types in {Self} cannot be translated to SQL"
+)]
 pub trait FunctionMetadata<A> {
     fn path() -> &'static str {
         core::any::type_name::<Self>()

pgrx-tests/tests/compile-fail/u32.stderr

@@ -1,11 +1,12 @@
-error[E0277]: the trait bound `fn(u32) -> u32: FunctionMetadata<_>` is not satisfied
+error[E0277]: cannot generate SQL schema for this function
  --> tests/compile-fail/u32.rs:6:5
   |
 5 | #[pg_extern]
   | ------------ in this procedural macro expansion
 6 | pub fn whatever(a: u32) -> u32 {
-  |     ^^ the trait `FunctionMetadata<_>` is not implemented for `fn(u32) -> u32`
+  |     ^^ some types in fn(u32) -> u32 cannot be translated to SQL
   |
+  = help: the trait `FunctionMetadata<_>` is not implemented for `fn(u32) -> u32`
   = note: this error originates in the attribute macro `pg_extern` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0599]: `u32` has no representation in SQL