make exception from "_process_result" a "no-result" log

so that it follows the doc and tests.

docs/usage.rst

@@ -182,7 +182,7 @@ from_pattern, to_pattern
 
 missing_ok
   Suppress warnings and errors if a version checking module finds nothing.
-  Currently only ``regex`` supports it.
+  Not all sources support it.
 
 proxy
   The HTTP proxy to use. The format is ``proto://host:port``, e.g.

nvchecker/core.py

@@ -444,7 +444,7 @@ async def process_result(
       if isinstance(r1, Exception):
         entry_waiter.set_exception(r.name, r1)
         # no versions are returned from "apply_list_options"?
-        logger.error('an error occurred', name=r.name, error=str(r1))
+        logger.error('no-result', name=r.name, error=str(r1))
         has_failures = True
         continue
       check_version_update(oldvers, r.name, r1, verbose)