New pool account where the funding amount + tx fee leaves less balance than open channels' anchor reserves causes pool to error on startup

If a user opens a new pool account, where the account funding amount + the onchain transaction fee leaves less balance than the current open channels anchor reserves, `pool` will accept the creation of the account but instantly error, as such a transaction is not allowed to be created by `lnd`.

Furthermore, when restarting `pool`, the startup process will error with:
`unable to start account manager: unable to resume account [string]: rpc error: code = Unknown desc = the outputs to be sent would leave insufficient reserves for anchor channels in the wallet`

This error will be repeated on every restart, until the user has deposited enough new onchain balance to exceed the total value of:
* The account funding amount
* The onchain transaction fee
* The total channel anchor reserves of all open channels

This was reported by a user through:
https://github.com/lightninglabs/lightning-terminal/issues/1193

-----

To resolve the issue for future users, we've discussed offline that we could potentially add the following functionality:

* We shouldn't make `pool` error on startup if this occurs, but rather log it as a warning.
* We could potentially add a 100k sats buffer, and not allow users to fund their pool accounts with the entire on-chain balance and require that the amount at least leaves out the 100k sats buffer.

Start with the pool account-creation and account-manager startup paths, then trace how lnd reports insufficient anchor-channel reserves. Reproduce the failure with funding below the required account, fee, and reserve total; done should cover the agreed startup-warning and minimum-funding behavior.

Written by the indexing model from the issue text.