Allow for exceptions thrown in goog.storage to be processed by an external error handler.

This allows for a centralized exception logging when setting/getting values, without having to wrap each call site with a try/catch block.

R=azzie,chrishenry
DELTA=227 (227 added, 0 deleted, 0 changed)


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MOE_MIGRATION=6081


git-svn-id: http://closure-library.googlecode.com/svn/trunk@2415 0b95b8e8-c90f-11de-9d4f-f947ee5921c8