More robust mapproxy-tiling - checks for empty output.

[ladislavhorky]

The output tileindex of mapproxy-tiling may end up empty for a variety of reasons - misconfiguration, bugs in other tools, etc. The empty tileindex is, however, always undesired output especially for users who are new to VTS as they get no error.

The mapproxy-tiling should exit with error whenever the resulting tileindex is empty. Moreover, logging some tileindex statistic if successful would be nice.

The same applies potentially for generatevrtwo: if the whole level of overviews if invalid, it should exit with error.