crash_sender: Properly handle curl errors

When the crash server returns an error code, correctly handle
it as a curl error.

Bug: 25295034
Change-Id: I75986a099cfcf90e5d7f2e9acf5616a164b5fc36

crash_reporter/crash_sender

@@ -446,7 +446,7 @@ send_crash() {
   local curl_stderr="${TMP_DIR}/curl_stderr"
 
   set +e
-  curl "${url}" -v ${proxy:+--proxy "$proxy"} \
+  curl "${url}" -f -v ${proxy:+--proxy "$proxy"} \
     --capath "${RESTRICTED_CERTIFICATES_PATH}" --ciphers HIGH \
     -F "prod=${product}" \
     -F "ver=${version}" \