cmd: remove default target

Let's handle errors before closing the issue.

Related to #354.
cmd: introduce undefinedTargetError

Related to #354.
cmd: change readTarget so it returns the target or an error

Related to #354.
fs/testing: returning ENOENT when reading files that doesn't exist
fs/testing: respect O_TRUNC
cmd: change GetUrl so it returns the URL or an error

Now that readTarget may return an error, GetUrl should raise this error.

Related to #354.
cmd/tsuru-base: handle cmd.GetUrl errors properly

Related to #354.
cmd/tsuru: handle cmd.GetUrl errors

Related to #354.
cmd/crane: handle cmd.GetUrl errors

Related to #354.
cmd/tsuru-base: fix tests

Closes #354.