Wrap the KeyError exception for missing variables

[amrishparmar]

The https://docs.python.org/3/library/string.html#string.Template.substitute method will throw a KeyError if values are missing, which is fine, but results in confusing behaviour for a user who sees an error message from the underlying implementation detail. This exception should be caught and have a friendlier message emitted from this library

Example of trace:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/amrish/miniconda3/envs/pedro2/lib/python3.11/site-packages/env_templating/templating.py", line 63, in update_environment_variables
    _write_env_file_with_substitutions(temp_output_file_path, template, substitutions)
  File "/home/amrish/miniconda3/envs/pedro2/lib/python3.11/site-packages/env_templating/templating.py", line 23, in _write_env_file_with_substitutions
    new_env_fp.write(env_template.substitute(substitutions))
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/amrish/miniconda3/envs/pedro2/lib/python3.11/string.py", line 121, in substitute
    return self.pattern.sub(convert, self.template)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/amrish/miniconda3/envs/pedro2/lib/python3.11/string.py", line 114, in convert
    return str(mapping[named])
               ~~~~~~~^^^^^^^
KeyError: 'DB_HOST'