Disable spongycastle before verifying jar files.

Fixes issue #111. This is a workaround rather than a full solution,
but it solves the problem. There is a bug in Lollipop, whereby including
spongycastle as a security provider causes problems verifying .jar files.
As a result, it is now disabled when actually performing verification,
so that the libraries provided by Android do the work, and then re-enabled
afterwards.

There is an ever so slight chance that the period when this is disabled
may align with the period when spongycastle may actually be required
(i.e. for signing a local repo index). This is a risk which I cnanot
see how to avoid, and will likel7 cause either the signing to fail
due to the unavailability of the relevant security classes. However
this is very minimal, hard to reproduce (I couldn't get it to happen)
and also has the effect of the local repo failing, rather than the updating
of apps failing (which is arguably more important) and so is worth it in
my opinion.

See comments at https://gitlab.com/fdroid/fdroidclient/issues/111
for much greater detail.
More robust failure conditions for local repo keystore.

Was experiencing some problems with local repo keystore on Android 2.3.4.
Previously, the local repo keystore could fail to create, but would still
return a LocalRepoKeyStore instance to be used. Now it throws a checked
exception, and is dealt with in the relveant places.

Also cleaned up some calls to "e.printStackTrace()" and replaced with
more informative logging messages.
Merge branch 'fix-111/lollipop-sadness' into 'master'

Fix issue #111 (Doesn't work on lollipop devices)

See Issue #111 (monster comment https://gitlab.com/fdroid/fdroidclient/issues/111#note_379713) for details of fix.

See merge request !41