ews: fix unnecessary reauthentication prompts

Without this change when OAuth credentials expired, two consecutive
requests could make the reauthentication state machine think that the
refresh token step failed and a full reauthentication is needed, while
in fact this wasn't the case.

With the change applied only the first failing request will trigger
the reauthentication process and any subsequent request failure will
be ignored until authentication is completed again.

Signed-off-by: Krzysztof Nowicki <krissn@op.pl>