Warn when a failed connection might be from a known bug

[jeffallen]

Fixes #657

[jeffallen]

The build failed due to a data race that seems to be associated with graceful:

WARNING: DATA RACE
314Read at 0x00c0001189f8 by goroutine 106:
315  net/http.(*conn).setState()
316      /home/travis/.gimme/versions/go1.15.linux.amd64/src/net/http/server.go:1753 +0x110
317  net/http.(*conn).serve.func1()
318      /home/travis/.gimme/versions/go1.15.linux.amd64/src/net/http/server.go:1806 +0xfd
319  net/http.(*conn).serve()
320      /home/travis/.gimme/versions/go1.15.linux.amd64/src/net/http/server.go:1945 +0xaff
321
322Previous write at 0x00c0001189f8 by goroutine 97:
323  gopkg.in/tylerb/graceful%2ev1.(*Server).manageConnections()
324      /home/travis/gopath/pkg/mod/gopkg.in/tylerb/graceful.v1@v1.2.15/graceful.go:399 +0x644
325
326Goroutine 106 (running) created at:
327  net/http.(*Server).Serve()
328      /home/travis/.gimme/versions/go1.15.linux.amd64/src/net/http/server.go:2969 +0x5d3
329  gopkg.in/tylerb/graceful%2ev1.(*Server).Serve()
330      /home/travis/gopath/pkg/mod/gopkg.in/tylerb/graceful.v1@v1.2.15/graceful.go:309 +0x472
331  gopkg.in/tylerb/graceful%2ev1.(*Server).ListenAndServe()
332      /home/travis/gopath/pkg/mod/gopkg.in/tylerb/graceful.v1@v1.2.15/graceful.go:147 +0xf2
333  go.dedis.ch/onet/v3.(*WebSocket).start.func1()
334      /home/travis/gopath/src/go.dedis.ch/onet/v3/websocket.go:194 +0x15c
335
336Goroutine 97 (finished) created at:
337  gopkg.in/tylerb/graceful%2ev1.(*Server).Serve()
338      /home/travis/gopath/pkg/mod/gopkg.in/tylerb/graceful.v1@v1.2.15/graceful.go:297 +0x377
339  gopkg.in/tylerb/graceful%2ev1.(*Server).ListenAndServe()
340      /home/travis/gopath/pkg/mod/gopkg.in/tylerb/graceful.v1@v1.2.15/graceful.go:147 +0xf2
341  go.dedis.ch/onet/v3.(*WebSocket).start.func1()
342      /home/travis/gopath/src/go.dedis.ch/onet/v3/websocket.go:194 +0x15c
343==================
344    testing.go:1023: race detected during execution of test
345--- FAIL: TestLocalTCPGenConnectableRoster (0.48s)
346=== CONT  
347    testing.go:1023: race detected during execution of test
348

We already have #601 reminding us to get rid of graceful, so I'm going to merge this anyway.