Add debug mode

This is badly needed for ties when grab doesn't do what you expect. It doesn't even look like it's running for me.

The Groovy Console does provide such feedback in the status bar.
But we've not added feedback for the groovy or groovysh commands.
People might be piping the output of a Groovy script to some other tool, and the Grab logging output might be interfering, so I'm not 100% sure it's a good idea. For groovysh this would be okay though I guess.

For reference, in the Groovy Console, we're installing an Ivy event logger here:
https://github.com/groovy/groovy-core/blob/master/subprojects/groovy-console/src/main/groovy/groovy/ui/Console.groovy#L226
And here:
https://github.com/groovy/groovy-core/blob/master/subprojects/groovy-console/src/main/groovy/groovy/ui/ConsoleIvyPlugin.groovy#L38

If they're piping output, can they turn all debuggin off with an option?

What release is this new feedback scheduled for?

I took a look at the source code (https://github.com/groovy/groovy-core/blob/master/src/main/groovy/grape/GrapeIvy.groovy#L369). There is already a debug mode. You just have to set the system property "groovy.grape.report.downloads" to "true" (e.g. add "-Dgroovy.grape.report.downloads=true" to JAVA_OPTS) and Grape will print the following infos to System.error:

• Starting to resolve a dependency
• Starting to download an artifact
• Retrying to download an artifact (Grape tries eight times)
• Download size and time for downloaded artifacts

You can use the system properties mentioned by Pascal already.
I'm closing the issue as "won't fix" as there's not more work to be done here since it's been already covered in an earlier version.