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.

---

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

.gitlab/pipeline/check.yml

@@ -107,7 +107,7 @@ check-rust-feature-propagation:
       echo "---------- Building ${PACKAGE} runtime ----------"
       time cargo build --release --locked -p "$PACKAGE" --features try-runtime
 
-      echo "---------- Executing `on-runtime-upgrade` for ${NETWORK} ----------"
+      echo "---------- Executing on-runtime-upgrade for ${NETWORK} ----------"
       time ./try-runtime \
           --runtime ./target/release/wbuild/"$PACKAGE"/"$WASM" \
           on-runtime-upgrade --checks=pre-and-post ${EXTRA_ARGS} live --uri ${URI}