Previously, one had to call `git-clone/fetch` directly with some
`-v` options in order to trigger verbose output of the remote helper.

With this change a DataLad log level of 'debug' (or more verbose),
also triggers verbose output in the helper. This helps with
accessibility of this feature for internal Git calls.

Note that due to a limitation of the `datalad -l` CLI option handling
the log-level is not propagated to subprocesses. This only works
for env specification ala DATALAD_LOG_LEVEL=debug

The `mirrorrepo` property of the datalad-annex Git remote helper
implementation creates an empty repository, if the remote has no
refs (or does not exist). This is sensible behavior for enabling
initial pushes.

However, it leads to successful but empty clones/fetches for any
misspecified URL or connectivity failure. This is undesirable.

This change adds an explicit test for an available/accessible
repository deposit, whenever a clone/fetch is performed. Wrong URL
specifications or connectivity issues will now lead to an explicit
error exit. This fact is communicated via a non-verbose message,
and the full error is available at verbosity level 2, i.e. with a
`git clone -v`.

Closes: dataladgh-636