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add support for checking if an ERL_NIF_TERM is an exception
Add the enif_is_exception function to allow callers to determine whether an ERL_NIF_TERM represents an exception. (Currently the only supported exception is badarg since only enif_make_badarg exists, but this will likely be expanded in future releases.) This allows NIF code to call other NIF functions that return ERL_NIF_TERM and properly check to see if the returned terms are exceptions. Without the enif_is_exception function, developers have to create their own means of checking whether a function creates an exception, such as returning boolean success/failure indicators or some other special value indicating that an exception is in effect. The declaration of enif_is_exception in erl_nif_api_funcs.h respects the order of declarations required to keep compatibility on Windows. Add a new test to verify the operation of enif_is_exception. Modify the erl_nif man page to add a description of enif_is_exception and also to clarify the requirements of calling the enif_make_badarg function. If code calls enif_make_badarg, the env passed in gets set with exception information and so the return value of the calling function MUST be the badarg term returned from enif_make_badarg. Also clarify that the result of enif_make_badarg may be passed only to enif_is_exception and not to any other NIF API functions.
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