Improve exception message in case the data set has not been solved yet

[triceo]

For the score analysis button, consider more friendly message in case the data set has not been solved yet. As of now, the message starts with "{"details":"Error id c9e674b0-daca-4829-8db8-326753a24402-2, org.jboss.resteasy.spi.UnhandledException ..." and only later, if one reads carefully, they learn what the root cause is.

For unexpected exceptions, it's ok. In this case, I would argues it's not, as halve the users will be tempted to click the score analysis button first.

Originally posted by @rsynek in #142 (comment)