When forwarding an HTLC and the outgoing channel doesn't have
enough funds or other transient issues, we are failing the
HTLC with a temporary channel failure.
From BOLT #04:
* if during forwarding to its receiving peer, an otherwise unspecified,
transient error occurs in the outgoing channel (e.g. channel capacity
reached, too many in-flight HTLCs, etc.):
-> return a temporary_channel_failure error.
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