remove 'this is a dummy struct' structs from generated erlang

the erlang backend adds 'this is a dummy struct' struct_info/1 (and struct_info_ext/1) clause to generated files which is probably incorrect behavior. it should probably instead be a function clause that catches all undefined structs and returns either 'undefined' or throws a 'function_clause' error. i've supplied a patch that does the latter (as that is the current behavior if you call struct_info/1 or struct_info_ext/1 with an undefined struct name)