RepoSense: Use better error display in frontend report

The generated report uses a generic placeholder for repositories that
failed analysis.

The placeholder looks similar to a blank rampchart, which may be
overlooked as it blends in with other rampcharts.

Also, the placeholder does not show useful errors to `Repo Owners` [1],
which may be useful for troubleshooting, as they may not necessary have
the CLI logs.

[1] CS2103/CS2113/CS3281 students