StanModule error handling

[gowerc]

If a file doesn't exist or if a string bit of code doesn't contain any core stan blocks (e.g. data { ...}) then the resulting Stan module will be blank. This is 9 times out of 10 a user error in that the file path is wrong or the stan code is broken. Ideally we should throw a warning if the user enters a non-empty string and gets an empty StanModule object back to get them to double check that this is intended.