Log startup errors to stderr

[n8maninger]

Makes it easier for scripts to parse startup issues

Fixes #203

[peterjan]

you missed two log.Fatal in main.go, there's also 3 calls to it in gen.go which I took with me in renterd since it's also potentially a command that needs to be run from a script

[n8maninger]

you missed two log.Fatal in main.go, there's also 3 calls to it in gen.go which I took with me in renterd since it's also potentially a command that needs to be run from a script

Did the ones in main.go, but I'm fine with the ones in gen.go since it's a build script.