SnapshotPOMHandler: show the file name upon errors

Unfortunately, this file name does not include the full path...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
MiniMaven: do not rely on the default handled for uncaught exceptions

Instead, show the stack trace and make sure that we exit with an error.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Ignore invalid maven-metadata-snapshot.xml files

There is a bug in MiniMaven (to be fixed soon) where it downloads
files to their final destination and leaves them there even if the
checksum verification failed. We should ignore such files.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Always download to temporary files before verifying the checksum

When downloading files together with their .sha1 files, we do verify
the checksums afterwards. But we do not remove the files (so that we
can investigate what went wrong later). So we need to download said
files to temporary files and rename them to the correct file names
only after verifying that everything's groovy.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Another Microsoft time tax

On Windows, of course, you have to spend an extra effort to
rename files. At the same time, atomically overwriting files is
impossible. Thanks so much, Bill Gates. You rock.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Use the caching URL of ImageJ's Maven mirror

ImageJ's Nexus server, responsible for serving and caching the Maven
repositories used by the various projects in the ImageJ ecosystem, has
several URLs to offer.

We used to access cached artifacts -- without looking at, say, OSS
Sonatype or Maven Central when the artifacts were not found.

Now we use the *caching* URL -- read: whenever artifacts are not found
in the cache, we let the Nexus server look actively in the upstream
repositories for our artifacts.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Some defensive programming

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Hard-code a couple more groupIds to help Fiji components build

Some Fiji projects still rely on properties for the groupIds of
certain components. Until the time this is fixed, let's bend over to
help poor Jarran Jenkins stay afloat.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge pull request #12 from scijava/download-fixes

Various download-related fixes