contracts migration: remove unnecessary panics (#2079)

Runtime migration CI is currently failing
(https://gitlab.parity.io/parity/mirrors/polkadot-sdk/builds/4122083)
for the contracts testnet due to unnecessary panicing in a `pre_upgrade`
hook.

Soon idempotency will be enforced
https://github.com/paritytech/try-runtime-cli/issues/42, in the mean
time we need to manually fix these issues as they arise.

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also removes backticks from the string in `echo`, which caused a
'command not found' error in ci output