lib: add CURL_WRITEFUNC_ERROR to signal write callback error #9874
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Prior to this change if the user wanted to signal an error from their write callbacks they would have to use logic to return a value different from the number of bytes (nmemb) passed to the callback. Also, the inclination of some users has been to just return 0 to signal error, which is incorrect as that may be the number of bytes passed to the callback.
To remedy this the user can now return CURL_WRITEFUNC_ERROR instead.
Ref: #9873
Closes #xxxx