Make better error message for TLS 1.5 fix

[ineiti]

The fix for go 1.15 with regard to TLS, #655, made the test-network crash. Some more information can be found in #656, which is, however, not capable of reproducing the error in a way that we can try to fix it.

So as the minimal service to users of our code, I propose we do the following:

Improve the error message given by an incompatible onet-connection and be more clear what might have happened. E.g., something like:

TLS handshake failed - this might be due to mismatching go-version numbers, e.g., 1.14 and 1.15, between nodes. Another reason might be nodes running with onet (3.2.4) that cannot communicate with onet (3.2.5)

We should also consider removing the 3.2.5 tag and replacing it with a 3.3.0 tag, and write something in the CHANGELOG.md