Improve log and exception message

[jugmac00]

When there is a newer transaction id on the client than on the server - for instance after a backup has been restored - following message gets logged:
"ClientStorageError: Client has seen newer transactions than server!"

The admin is left with this message and probably does not know how to recover a working state - as Zope is no longer usable.

What worked for me - and maybe is not always the best solution was to remove the zec file, cf
https://docs.plone.org/3/en/manage/troubleshooting/transactions.html#zeostorage-client-has-seen-newer-transactions-than-server

There may be other reasons for this error and maybe other recovery steps make more sense then.

From a quick glance tox is not setup to run coverage (see #123 ), and from looking at the code and the tests it seems the branch ("Cache newer than server") is untested.

Update
The branch gets covered by

ZEO/src/ZEO/asyncio/tests.py

Line 437 in a6c1ffc

def test_bad_server_tid(self):

Was hard to spot, as the test does neither check for the AssertionError nor for the log message.

ZEO/src/ZEO/asyncio/client.py

Lines 488 to 493 in bd7cd06

	elif cache_tid > server_tid:
	self.verify_result = "Cache newer than server"
	logger.critical(
	'Client has seen newer transactions than server!')
	raise AssertionError("Server behind client, %r < %r, %s",
	server_tid, cache_tid, protocol)

Also, the tests for ZEO seems to be quite flaky, cf #140

@jamadden made some good suggestions how to proceed:
#139 (comment)

[Removed wrong assumption about assert / AssertionError]

[jamadden]

I do not know what to think about the AssertionError - as there is no guarantee that it get raised,

It will always be raised. Only the assert statement can be disabled by -O; a raise is always executed.