[runtime]When setting a loader error, clear the previous one to avoid the leak.
[runtime]Type parsing of generics should not rely on the presence of a backtick. Fixes #15124.

We previously depended on the presence of a backtick in a generic type name to try to
parse a generics argument list. This is not what the spec says and F# breaks this expectation.

The right way to handle this is to probe for array syntax.
Add some tests for #15124.