Log error message if no documents are found in the configured directory.

Log error message if no documents are found in the configured directory.

* src/main/java/org/glom/web/server/OnlineGlomServiceImpl.java: Also:
Extract the glom file extension string to a private static final class
variable (mostly as syntactic sugar). Accept a minor formatting change.
* src/main/resources/onlineglom.properties: Add '/glomfile' to end of
the example glom.document.directory configuration property to make it
more clear that it can any directory, not just the home directory.