use FailureDetail enum for htlc event failures

[dougEfresh]

LinkFailEvent.FailureString is an arbitrary string of undefined length.

I noticed in my Prometheus a feeinsufficient failure caused a very large label value.

The Enum of FailureDetail has a short name describing the failure.

failure_reason="feeinsufficient(htlc_amt==xxxxxxxx,_update=(lnwire.channelupdate)_{ 
_signature:_(lnwire.sig)_(len=64_cap=64)_{ __00000000__xxxxxx__|xxxxxx.| __00000010_xxxxxxxx__|o&.i...h..qj.{..| 
__00000020__xxxxxx&]3'...ea...h..<| __00000030__25_18_98_e4_50_cd_c1_1a__ff_f0_bf_b9_b5_08_67_cf__|%...p.........g.| _}, _chainhash:_(chainhash.hash)_(len=32_cap=32)xxxxxx, _shortchannelid:_(lnwire.shortchannelid)_xxxxxx, 
_timestamp:_(uint32)_1646318182, _messageflags:_(lnwire.chanupdatemsgflags)_00000001, _channelflags:_(lnwire.chanupdatechanflags)_00000000, _timelockdelta:_(uint16)_40, 
_htlcminimummsat:_(lnwire.millisatoshi)_1000_msat, _basefee:_(uint32)_0, _feerate:_(uint32)_50, 
_htlcmaximummsat:_(lnwire.millisatoshi)_1600000000_msat, _extraopaquedata:_(lnwire.extraopaquedata)_{ _} } ",