Harvesting - give more info to user why he can't start harvesting

[cryptoBeliever]

Currently user can't start harvesting when:

• balance below 10000 XYM (message "Insufficient balance, the account needs to hold at least 10000 XYM to start delegated harvesting")
• importnace = 0 (message "Your account cannot start delegated harvesting while importance is equal to 0")

Improvements:

• we should change message for importance to "Your account cannot start delegated harvesting while importance is equal to 0. Importance recalculation will take up to 12h."
• in both cases we should also link user to https://docs.symbolplatform.com/guides/harvesting/activating-delegated-harvesting-wallet.html#prerequisites

[orisdorch]

I think it's also worth providing more detail on the Failed status. Right now it's stated as "Your delegated harvesting request has failed, please stop harvesting and try a different node." -- if there is a possibility that the wallet is misreporting the failure, and the harvesting status is actually active, but the wallet is unable to retrieve this for some reason (i.e. harvesting on a peer node with no API, outage of the symbol.services site, etc), then more detail would be useful here.