fix: azure-curl provider exception type leaks to through core apis to customer!

built on top of MR !91 (merged) that allows upgrade to recent libxml2 fixes the build of experimental providers.

This is a workaround.

The correct things to consider doing are :

• make -1LL a constant for Storage::MISSING_OBJECT_SIZE or the like
• return that instead of an exception in all providers
• make sure no provider dependent exception type leaks. the customer must not have to write new error handling code when changing cloud provider.
• potentially return stack trace in exception messages...

( current code just sent "Blob does not exist" message. to a call trying to write a new blob... )