[runtime] Fix MonoError lifecycle in do_try_exec_main (#3145)

[runtime] Fix MonoError lifecycle in do_try_exec_main (#3145)

The do_try_exec_main function is always called in contexts where any
exception from doing the invoke is going to be stored in the exc outarg.
That means any MonoError will be handled locally by do_try_exec_main.
So we don't need a MonoError argument, we just need an inner_error.