Sling Models: Allow caller to deal with exceptions

Currently due to the specification of the adaptTo-method to return null if adaptation is not possible, the caller is not notified about any exceptions (because they are caught within the ModelAdapterFactory).

This is e.g. necessary to deal with validation exceptions properly (i.e. required field injection not possible). The problem was also discussed briefly in http://apache-sling.73963.n3.nabble.com/Silng-Models-Validation-Framework-td4033411.html.

All exceptions either being thrown by the
@PostConstruct method or caused by the field/method injection are not propagated but basically swallowed by Sling Models.

It would be great to be able to catch those exceptions either in the view or in a servlet filter. I think it should be possible to throw unchecked exceptions in the ModelAdapterFactory.getFactory() method if it is requested (i.e. through a global OSGi configuration flag for Sling Models).
WDYT?
Would you accept such a patch or do you think this breaks the API (also compare with https://issues.apache.org/jira/browse/SLING-2712?focusedCommentId=13561516&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13561516).

If it does not work through the adaptTo, SlingModels should provide an alternative way of instanciating models (and propagating exceptions), although this is kind of tricky, because it internally relies on adaptTo as well (e.g. in https://github.com/apache/sling/blob/trunk/bundles/extensions/models/impl/src/main/java/org/apache/sling/models/impl/ModelAdapterFactory.java#L647)