Fixes #160

If the front-end runs into a type it doesn't understand in the parameter list of an entry point, it will create an `ErrorType` for that parameter, but then the parameter binding/layout rules will fail to create a `TypeLayout` for the prameter (and return `NULL`).
There were some places where the code was expecting that operation to succeed unconditionally, and so would crash when there was a bad type.

The specific case in the bug report was when the return type of a shader entry point was bad:

    // `vec4` is not an HLSL type
    vec4 main(...) { ... }

Note that the specific case in the buf report only manifests in "rewriter" mode (when the Slang compiler isn't allowed to issue error messages from the front-end), but the same basic thing would happen if the varying parameter/output had used a type that is invalid for varying input/output:

    Texture2D main(...) { ... }

I'm not 100% happy with just adding more `NULL` checks for this, because there is no easy way to tell if they are exhaustive.
A better solution in the longer term might be to construct a kind of `ErrorTypeLayout` to represent cases where we wanted a type layout, but none could be constructed.