When the MT detects an attempt to disconnect the last PDN or when the
network returns a response message with cause value #49, the "Last PDN
disconnection not allowed" error is returned.
The numeric error code was changed from 151 to 171 in 3GPP Rel-11, and
therefore there are devices out there that would conform to Rel-10 or
below and that would report +CME ERROR code 151 instead of 171. Given
that 151 isn't defined to a different meaning in the specs, let's
define it in the same way as 171.
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