Confusing error message when enabling --watchonlywallet and --flatutreexoproofindex

[kcalvinalvin]

--flatutreexproofindex enables --noutreexo internally but it doesn't alert the user of doing so. When a user tries to enable --watchonlywallet it errors out and gives a confusing error message:

loadConfig: the --watchonlywallet requires the --noutreexo option off

[1amhesus]

Hello @kcalvinalvin,

I was looking into the code, specifically lines 1217-1222(config.go), where the current error message that appears when using the --watchonlywallet option could potentially be confusing for users because users can't recognize these two option conflict. Firstly, I suggest that some works on improving this error message to make it clearer with aditing README.md like below,

Bridge nodes are nodes that keep the entire merkle forest and attach proofs to new blocks and transactions. Since miners and nodes publish blocks and transactions without proofs, these nodes are needed to allow for utreexo nodes without a soft fork. To run a bridge node:

# Either one will work. The only difference these have is that the flatutreexoproofindex
# stores all the data in .dat files instead of leveldb. It makes it easier to read the
# proofs for tinkering.
# Note: The --flatutreexoproofindex option automatically enables --noutreexo. 
# If you use the --watchonlywallet option, it will conflict with --noutreexo and cause an error.
`./utreexod --flatutreexoproofindex`
`./utreexod --utreexoproofindex`

# For running bridge nodes that are also archival.
# Note: The --flatutreexoproofindex option automatically enables --noutreexo. 
# If you use the --watchonlywallet option, it will conflict with --noutreexo and cause an error.
`./utreexod --flatutreexoproofindex --prune=0`
`./utreexod --utreexoproofindex --prune=0`

Additionally, in lines 1199-1207(config.go), the --flatutreexoproofindex option automatically enables --noutreexo. It might be helpful to add a notification to inform the user about this internal activation.

Lastly, I’m curious to know if you have any thoughts on restructuring the relationship between these conflicting options. Do you feel that this is something necessary to do immediately?