Prompted by the test suite on windows failing to with "export foo failed"
and no information about what went wrong.
Note that only storeExportWithContentIdentifier has been converted.
storeExport still returns a Bool and so exceptions may be hidden.
However, storeExportWithContentIdentifier has many more failure modes,
since it needs to avoid overwriting modified files. So it's more
important it have better error display.
When a remote is configured to be readonly, don't allow changing what's
exported to it.
This was missed in the original export remote implementation, but it makes
sense for a readonly export remote to not be allowed to change.
This does not change the overall license of the git-annex program, which
was already AGPL due to a number of sources files being AGPL already.
Legally speaking, I'm adding a new license under which these files are
now available; I already released their current contents under the GPL
license. Now they're dual licensed GPL and AGPL. However, I intend
for all my future changes to these files to only be released under the
AGPL license, and I won't be tracking the dual licensing status, so I'm
simply changing the license statement to say it's AGPL.
(In some cases, others wrote parts of the code of a file and released it
under the GPL; but in all cases I have contributed a significant portion
of the code in each file and it's that code that is getting the AGPL
license; the GPL license of other contributors allows combining with
AGPL code.)
