pointed out by @felixfontein in #222, we shouldn't delete the challenge files on errors, but we should clean them up on success

acme_tiny.py

@@ -140,7 +140,6 @@ def get_crt(account_key, csr, acme_dir, log=LOGGER, CA=DEFAULT_CA, disable_check
             wellknown_url = "http://{0}/.well-known/acme-challenge/{1}".format(domain, token)
             assert (disable_check or _do_request(wellknown_url)[0] == keyauthorization)
         except (AssertionError, ValueError) as e:
-            os.remove(wellknown_path)
             raise ValueError("Wrote file to {0}, but couldn't download {1}: {2}".format(wellknown_path, wellknown_url, e))
 
         # say the challenge is done
@@ -148,6 +147,7 @@ def get_crt(account_key, csr, acme_dir, log=LOGGER, CA=DEFAULT_CA, disable_check
         authorization = _poll_until_not(auth_url, ["pending"], "Error checking challenge status for {0}".format(domain))
         if authorization['status'] != "valid":
             raise ValueError("Challenge did not pass for {0}: {1}".format(domain, authorization))
+        os.remove(wellknown_path)
         log.info("{0} verified!".format(domain))
 
     # finalize the order with the csr