codegen: Use copies of error_variable of catch-clauses for nested usage
codegen: Use copies of error_variable of catch-clauses for nested usage

Calling visit_local_variable() multiple times on the same instance of a
LocalVariable causes issues.

frida/vala,frida/vala,GNOME/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala,frida/vala,GNOME/vala