Add warning about missing max_cj_fee_rel/max_cj_fee_abs settings

[theborakompanioni]

From @AdamISZ

i encountered a tricky error, but it's a function of me as user: i got 'error, config var not present' when starting the coinjoin. it took me a while to track down that it was because max_cj_fee_rel/abs was not present that caused it. users will have default joinmarket.cfgs at least to start with, so i hope they won't see that problem as they won't know how to fix it.

In the standalone versions of Jam the config variables max_cj_fee_rel and max_cj_fee_abs are populated (e.g. see Umbrel and Citadel). As these variables have no default values, users connecting to an existing jmwalletd instance will only ever see the message config var not present if they are missing. As a user I want a meaningful message about what is wrong or what I can do about it if manual adaptions are required.

Easiest workaround for that would be to always try to retrieve these values first.
It'd be also an option to set these variables according to user input, but these changes would not be persistent.
Assuming a user connects to an already running daemon, he or she is capable of adapting the config file as well.