Added handling of onInstallerBeforePackageDownload event responses.

Pull Request for Issue # .

Summary of Changes

Allow the following events to return an empty url:

• onInstallerBeforePackageDownload
• onInstallerBeforeUpdateSiteDownload

Added the following error handling event:

• onInstallerPackageDownloadFailed

Testing Instructions

Install the attached plg_installation_test.zip
The code explains how to use the new functionality.
Enable the plugin to cause all extension updates to fail and illustrate the resulting error messages.

Actual result BEFORE applying this Pull Request

N/A

Expected result AFTER applying this Pull Request

The new behaviour as described in testing instructions.
All existing plg_installation_**** plugins behave as previous.
plg_installation_test.zip

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• [*] No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• [* ] No documentation changes for manual.joomla.org needed

Recommend adding instructions to manual.joomla.org to describe these features.
Derive these documentaion changes from the comments in plg_installation_test .