Missing `type` attribute for a canister causes panic

Missing type attribute for a canister causes panic (should cause a regular error).

$  dfx start --background --clean && time dfx deploy -vv
Running dfx start for version 0.20.0-icsystemtest.0+rev256.f4913345
Using project-specific network 'local' defined in /home/porton/Projects/zondirectory2/dfx.json
  Version v0.20.0-icsystemtest.0+rev256.f4913345 installed successfully.
Initialized replica.
Dashboard: http://localhost:36121/_/dashboard
Trace mode enabled. Lots of logs coming up.
Checking if identity 'default' exists.
Building dependencies graph.
thread 'main' panicked at src/dfx/src/lib/builders/motoko.rs:44:96:
called `Result::unwrap()` on an `Err` value: Failed to create <Type>CanisterInfo for canister 'frontend'.

Caused by:
    `main` attribute is required on Motoko canisters in dfx.json
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Aborted (core dumped)

real	0m15.212s
user	0m16.274s
sys	0m1.671s

Start with src/dfx/src/lib/builders/motoko.rs at the reported line and reproduce the failure with dfx start --background --clean followed by dfx deploy -vv using a canister missing its type attribute. Replace the panic behavior with a regular error and verify that deployment reports the configuration problem without aborting from an unwrap.