Do not fail on missing descriptors

Assembly requires too much boilerplate right now to be used in a reactor based project because of the way it fails out if no descriptors are found. I suggest adding a boolean parameter (probably ignoreMissingDescriptors) which can default to false for backwards compatability but when set to true, does nothing if no descriptors are found.

Then in org.apache.maven.plugin.assembly.io.DefaultAssemblyReader line 131 add the check for that new parameter so that it only fails out if it was configured to fail out. Otherwise it just goes about its business of assembly nothing (which seems like a perfectly reasonable logic as well).

By doing these things, it makes it possible to configure the assembly plugin 1 time in a parent pom and then not have to configure it in several (in my case 11 out of 19) different modules again and again.