Channel open error messages need clearer language

[myxmaster]

Describe your enhancement idea

I found those 2 when trying to open a channel on testnet:

-> Pretty bad language and not easy (impossible?) to understand for beginners.

[myxmaster]

I made a PR in lnd to fix the 2nd error message. The first one is maybe not so bad, I was just confused because I thought I had enough testnet funds and therefore thought something else is meant. Closing for now (in case lnd doesnt merge, we can still override).