Improve error handling

[humphd]

In various places you write raw error objects to the user, eg. https://github.com/Reza9472/StaticSiteGenerator/blob/main/index.js#L123.

A better solution is to give a useful error message, with details on what went wrong and how to fix, and then exit the program with a non-zero error code:

if(err) {
  console.error('Useful error message here with instructions to fix...');
  process.exit(-1);
}