[BUG] XAR installation with dependencies may fail with unclear error

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Describe the bug

The installation of a package will fail with an unclear error message "Package descriptor not found" when loading of a dependency fails due to a connection problem with the package repository. At the time of writing this happens when trying to download dependencies from the package repository on exist-db.org due to an HTTPS downgrade (see https://github.com/eXist-db/public-repo/issues/74).

Expected behavior

A clear and concise error message stating that the repository was not available, misconfigured, which dependency could not be downloaded and from where.

The error should likely be handled in Deploy#load but is not caught at that point.

To Reproduce

1. set the package repository URL in your packageservice to https://exist-db.org/exist/apps/public-repo
2. try to install any package that declares a dependency that is not yet met
3. See error

Context (please always complete the following information):

• OS: macOS 10.15.7
• eXist-db version: 6.0.0
• Java Version Java8u312

Additional context

• How is eXist-db installed? built from source tags/eXist-6.0.0
• Any custom changes in e.g. conf.xml? none

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related https://github.com/eXist-db/public-repo/pull/76