In case there is not enough quota left to create the trace buffer
or trace policy dataspace throw Out_of_metadata explicitly instead
of rethrowing the Ram_session::Quota_exceeded exception. Now one
can catch Trace::Out_of_metadata exception in a client application.
In addition fix Allocator_guard::withdraw() checks because this
method does not throw any exceptions and a failed withdrawal goes
unoticed.
Fixes#871.
Josef Söntgen
Norman Feske