Report generator: Link to result browser dirs only if there is an index.html

[jowerner]

When generating the Errors page, the report generator may add a result browser link to an error table entry. It does this if it was able to find a corresponding result browser directory.

However, sometimes test suites store additional data files in the result browser directory for later analysis. This causes the report generator to link to this directory, even if this directory does not contain result browser files (due to the error limiter). Clicking on this link would then result in a Not Found page because there is no index.html file. Only if such an index.html file exists, should the report generator link to this directory.