refactor: Improve error message when try to deploy with Sol metadata

[alexbcasas]

Issue Reported During Web3 Summit Hackathon

If I build a contract with Solidity metadata

[package.metadata.ink-lang]
abi = "sol"

And try to deploy it:

cargo contract instantiate --suri //Alice --args false

The error message is a bit confusing:

ERROR: Failed to deserialize metadata file /Users/alexbean/contracts/my_contract/target/ink/my_contract.json

Caused by:
    missing field `source` at line 1 column 5764