A value of 1 means that curl should fail silently if the HTTP status code is equal to or larger than 400. The default action would be to return the page normally, ignoring that code.
This method is not fail-safe, and there are scenarios where non-successful response codes will slip through. For more information see the documentation of libcurl (http://curl.haxx.se/).
local(c = curl('www.example.com/login')) #c->set(curlopt_failonerror, 1) #c->result
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