rdbms: Replace reportConnectionError() with direct throws

rdbms: Replace reportConnectionError() with direct throws

When reading through DatabaseMysqlBase::open(), it was not
obvious that execution would not continue after the conditional
`!$this->conn` block, given it ends in a method call, without
return or throw. I considered adding a return statement after it
for clarity, but it seems in this case it might make more sense
to throw directly given $error here has already gone through a
fallback to getLastError() a few lines up.

Replace the other three calls to reportConnectionError() as well,
which previously passed a useful string that was overwritten
with lastError(). Instead, log both. And make their call to
queryLogger->error() match the previous ones to have an 'error' as well.

This leaves reportConnectionError() as being unused, except for
a call from LoadBalancer. That call was problematic because
it was inside a conditional for IDatabase, but the method isn't
part of that interface. Replace it with a direct throw as well.

Deprecate the method as its now unused in core, and also remove its
'# New method' comment which hasn't made sense since r75341 (16cded8b32).

Change-Id: I0f2ef00ba44bf7090a3ce54edeb8c7e8e543e46a