Error level logs should be printed by default

[matthiasgeihs]

It seems like if no logger is set up, runtime errors happening in the backend are not visible. This is problematic because relevant information might be hidden from a user. For example, when the funder fails to perform a channel funding operation, the user will just see "did not fund channel in time", while the logger actually has some more information "out of gas" or "failed to increase allowance".

I suggest that we make such error level logs visible to the user by default, for example, by printing them to stdout if no logger is set.

[ggwpez]

I like the idea. I set the default log level in the client package to warn now.
After testing it with the tutorial app, this seems to print funding errors. Maybe you can also test it?
Code is in #66