`appEntryPoint` is not validated, relies on Chromium errors

[goosewobbler]

We can avoid some weird Chromium errors by checking if a provided appEntryPoint exists when it is specified, and throwing a nice informative Error.

This will need rationalising with the edge case where both appEntryPoint and appBinaryPath are provided. Currently we ignore appBinaryPath if appEntryPoint is specified but we will be introducing additional state(s), e.g. where both are provided and appBinaryPath is valid but appEntryPoint is not.