Improve error message for conflicting priorities
Improve error message for conflicting priorities

I find the error message 'nix-env --set-flag priority NUMBER PKGNAME'
not as helpful as it could be :
- doesn't share the current priorities
- doesn't say that the command must be run on the already installed
PKGNAME (which is confusing the first time)
- the doc needs careful reading:
"If there are multiple derivations matching a name in args that have the same name (e.g., gcc-3.3.6 and gcc-4.1.1), then the derivation with the highest priority is used."
if one stops reading there, he is screwed. Salvation comes with reading "A derivation can define a priority by declaring the meta.priority attribute. This attribute should be a number, with a higher value denoting a lower priority. The default priority is 0."
To sum it up, lower number wins. I tried to convey this idea in the
message too.

layus/nix,vcunat/nix,peti/nix,layus/nix,dezgeg/nix,zimbatm/nix,peti/nix,shlevy/nix,ehmry/nix,ehmry/nix,vcunat/nix,rycee/nix,dezgeg/nix,peti/nix,dtzWill/nix,rycee/nix,layus/nix,peti/nix,NixOS/nix,pikajude/nix,dtzWill/nix,dezgeg/nix,rycee/nix,NixOS/nix,zimbatm/nix,dtzWill/nix,dezgeg/nix,rycee/nix,layus/nix,NixOS/nix,peti/nix,ehmry/nix,ehmry/nix,zimbatm/nix,shlevy/nix,pikajude/nix,rycee/nix,vcunat/nix,shlevy/nix,dtzWill/nix,layus/nix,shlevy/nix,dezgeg/nix,vcunat/nix,zimbatm/nix,dtzWill/nix,ehmry/nix,pikajude/nix,ehmry/nix,shlevy/nix,NixOS/nix,NixOS/nix