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."