When the package at `CodeUri` does not exist, report a warning

### Describe your idea/feature/enhancement

When using `sam local start-lambda`, `invoke` or `start-api`, the output describes the activities between the local machine and the lambci container while processing the invocation. When the package is present and a zip/jar, then we even see it being de-compressed before mounting.

However, I've been caught out a few times by sam quietly just mounting the `CodeUri` path in the target container when that path does not exist on the local machine. This results in an error which is secondary to the problem.

### Proposal

While detecting if the `CodeUri` represents something to unzip/unpack, output a warning if the `CodeUri` appears to refer to a non-existent path. This will speed up discovery of this common error.

### Additional Details