Improve error message for "failed to parse start time"

Report the actual start_time value that couldn't be parsed so it's
clearer when gcalcli is doing something silly like interpreting
--calendar=Foo as a positional arg (see #697).

gcalcli/gcal.py

@@ -1238,7 +1238,7 @@ def CalQuery(self, cmd, start_text='', count=1):
                 start = start.replace(hour=0, minute=0, second=0,
                                       microsecond=0)
             except Exception:
-                self.printer.err_msg('Error: failed to parse start time\n')
+                self.printer.err_msg(f'Error: failed to parse start time {start_text!r}\n')
                 return
 
         # convert start date to the beginning of the week or month