read_yaml fails when it tries to expand an environment variable which contains a $

[sentry-io]

Sentry Issue: RASA-OPEN-SOURCE-1XP

ValueError: Error when trying to expand the environment variables in '${DB_PASSWORD}'. Please make sure to also set these environment variables: '['$dsfdsfwece2s3f']'.
(13 additional frame(s) were not displayed)
...
  File "rasa/utils/endpoints.py", line 26, in read_endpoint_config
    content = rasa.shared.utils.io.read_config_file(filename)
  File "rasa/shared/utils/io.py", line 527, in read_config_file
    content = read_yaml_file(filename)
  File "rasa/shared/utils/io.py", line 368, in read_yaml_file
    return read_yaml(read_file(filename, DEFAULT_ENCODING))
  File "rasa/shared/utils/io.py", line 349, in read_yaml
    return yaml_parser.load(content) or {}
  File "rasa/shared/utils/io.py", line 314, in env_var_constructor
    " variables: '{}'.".format(value, not_expanded)

To reproduce:

import os
from rasa.shared.utils.io import read_yaml

os.environ['HELLO'] = '$sdfdsf'

print(read_yaml("""
hello:
- ${HELLO}
"""))

[sentry-io]

Sentry issue: RASA-OPEN-SOURCE-12E

This is an example of when it is a user error and should be handled accordingly

[sentry-io]

Sentry issue: RASA-OPEN-SOURCE-665

[wochinge]

I think we have to make it a RasaException as this still raises a ValueError in case the user doesn't supply an env variable

[sentry-io]

Sentry issue: RASA-OPEN-SOURCE-8SV