Improve the error message when libdoc fails to generate documentation

Prior to RF 7.5 the following command runs successfully:

libdoc Process Process.json

With RF 7.5 the command fails with the error

Markdown format requires 'markdown' module to be installed.

This error arises because:

• in RF 7.5 the documentation of the standard RF libraries is written in Markdown
• when the output format is JSON by default libdoc converts the documentation to HTML
• the 'markdown' module is required in order to convert Markdown to HTML

The current error message doesn't make clear why the 'markdown' module is required. It comes from a leaky abstraction.

My suggestion would be to introduce pre-conditions (via assertions) in each stage of the pipeline for generating the documentation. For example, in the stage that converts the documentation to HTML:

def module_exists(module: str) -> bool:
   try:
      import_module(module)
      return True
   except ModuleNotFoundError:
      return False

def convert_to_html(doc) -> str:
    match doc.format:
         case 'markdown':
                assert module_exists('markdown'), "The 'markdown' module is required to convert Markdown documentation to HTML"
                return md_to_html(doc)
         case 'robot':
               return robot_to_html(doc)