Add descriptive adl errors
Merge remote-tracking branch 'deba12/master'
adl: rename adl_errors() to adl_error_desc(), shorten error desc's, handle unknown error
core: change default scantime/expiry to 7/28 (back to 1:4 ratio).

Previous defaults (before 10/30) were optimised for Bitcoin that has
a much higher average block time. Optimise for Litecoin (currently
largest scrypt network.)

Not pushing it further down so as not to scare people with dropping
hashrates.

Thanks to MissedOutOnBTC @ freenode for testing.

Closes https://github.com/veox/sgminer/issues/22