Improve handling of unavailable IERS-B (predictive future Earth rotation) data
in two ways. First, allow conversions with degraded accuracy if the IERS-B data
are missing or do not cover the required time span. This is done with a new
config item ``conf.iers_degraded_accuracy`` which specifies the behavior when
times are outside the range of IERS table. The options are 'error' (default),
'warn' (issue a ``IERSDegradedAccuracyWarning``) or 'ignore' (ignore the
problem). Second, the logic for auto-downloads was changed to guarantee that no
matter what happens with the IERS download operations, only warnings will be
issued.

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