[DSP-52] Shorten strings in UStore

Description

Problem: we have some issues with size of code transpiled from Lorentz due to some limitations of Tezos. Code which uses UStore extensively often has cases which can fail with rather long strings. They can be made shorter without loss of information and readability and save some space.

Solution: shorten some error messages.

Related issue(s)

https://issues.serokell.io/issue/DSP-52

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