Improve handling of the home directory creation.

This commit improves a couple of issues surrounding the creation of the
btcd home directory.

First, the code was previously attempting to log any errors that occurred
while creating the directory using the logging system which is not
initialized at that point.  Thus, nothing was displayed to the user.

Second, if any component of btcd home directory path already exists, but
is not a directory, such as in the case of symlinks, the error returned
from the os.MkDirAll call indicates the directory can't be created.  While
this is true, it's not always the most helpful error to display to the
user.  So, this commit adds logic to detect when the failure case is due
to an existing symlink and displays a nicer error message suggesting the
user check if the destination of the link is mounted.

Fixes #193.