add debug info to storage_description

Fix #69 (closed) : adding debug information to error raised in storage_description

A couple of examples of the new errors messages:

• "An indexed subcontext at [delegates_with_frozen_balance / Index of block_cycle] already exists but has a different argument: pkh <> Baker_hash."
• "Could not register a value at [contracts / index / Index of contract_id / delegated / Index of contract_id] because of an existing Value."

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Co-Authored-By: Tomáš Zemanovič tzemanovic@gmail.com