Spawn failures end with a misleading 'cannot infer a schema' error #1187

When every file's parser fails to spawn (e.g. --on-file extract.py with the script not on $PATH), dirsql reports the spawn error per file and then ends with "parser produced no rows ... cannot infer a schema". The real cause is the spawn failure; the schema line points the user the wrong way.

Expected: when no file produced rows because every parser failed to spawn, the final error names the spawn failure, not schema inference.

Split out of #1172 because it changes behavior, not just wording.