Fix error when running tests

The required namespace name is no longer right. It was renamed.
Remove spurious check

It always returned true.
Fail fast if calling erroneously close-watcher

Now close-watcher can only be called on objects returned by watch-dir.
Otherwise you could call close-watcher in an unrelated agent causing it
to enter in a fail mode.
Make dirwatch more resilient

If the reporting function `f` throws an exception the agent used would
enter in fail mode and stop polling for changes.

Now if an exception occurs in `f`, the exception is reported and the
agent keeps working.
Now the default mode is continue

If an exception happened inside wait-for-events the agent would not
enter in an error state.

No situation where an exception in wait-for-events was found, but better
be safe.
Discard overflow events

Couldn't reproduce a case on which an overflow event was produced, but
in the case it happened the event is discarded.
Move f to top let

Just a simple refactoring.
Some cleanups
Merge pull request #8 from ogf/some-fixes

Several fixes and improvements