Add error checking for target extensions

[rynowak]

Right now we don't check for the absence of target extensions when we render an extension node. This means that we could null-ref instead of reporting an error.

[rynowak]

Here's an example of what this looks like https://github.com/aspnet/Razor/pull/1357/files#diff-9c6d50b13b132c9feed9b3f0e7becd43R31

[rynowak]

17800b6