Add a distinct exception for missing environment variables

Description

EnvError currently represents both a missing required environment variable and the base class for validation failures:

class EnvError(ValueError):
    ...

class EnvValidationError(EnvError):
    ...

This makes it impossible to catch only the “variable is not set” case without also suppressing malformed values.

For example:

from environs import EnvError, env

try:
    database = env.dj_db_url("DATABASE_URL")
except EnvError:
    database = None

This handles an absent DATABASE_URL, but also silently catches an invalid value such as:

DATABASE_URL=not-a-database-url

env.dj_db_url() raises EnvValidationError, which is also an EnvError. Applications wanting optional configuration can therefore discard useful validation errors and fail later with less informative errors.

Proposed solution

Introduce a specific exception for missing variables, for example:

class EnvError(ValueError):
    pass


class EnvNotSetError(EnvError):
    pass


class EnvValidationError(EnvError):
    pass

Then raise EnvNotSetError when a required variable is absent:

try:
    database = env.dj_db_url("DATABASE_URL")
except EnvNotSetError:
    database = None

This should be backward-compatible for callers already catching EnvError, while allowing callers to distinguish missing configuration from invalid configuration.

Reproduction

import os

from environs import EnvError, env

os.environ.pop("DATABASE_URL", None)

try:
    env.dj_db_url("DATABASE_URL")
except Exception as error:
    print(type(error).__name__)
    # prints EnvError

os.environ["DATABASE_URL"] = "not-a-database-url"

try:
    env.dj_db_url("DATABASE_URL")
except Exception as error:
    print(type(error).__name__)  # prints EnvValidationError
    print(isinstance(error, EnvError))  # prints True

Observed with environs==15.0.1.