Extend placeholder validation to show file where placeholder must be located

[dshulyak]

After https://github.com/tendermint/starport/pull/1150 is merged every command will print error if placeholders are missing:

Missing placeholders:

// this line is used by starport scaffolding # stargate/testutil/appArgumen

Some placeholder are used more than once, so it might be confusing to find the root issue.

[itsdevbear]

bump, this would be very helpful

[aljo242]

Closing this issue because we are currently refactoring the placeholder system. It will be removed and replaced with a more robust AST inspection system.