Change sys.exit to raise ImportError (#2440)

The fix for #1688 in #1761 breaks help("modules") introspection and also leads
to unhappy results when inadvertently importing blackd from Python. Basically
the sys.exit(-1) causes the whole Python REPL to exit -- not great to suffice.

Commit history before merge:

* Change sys.exit to Raise.
* Add #2440 to changelog.
* Fix lint error from prettier
* Remove exception chain for more helpful user message.

Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>

CHANGES.md

@@ -10,6 +10,10 @@
 - The failsafe for accidentally added backslashes in f-string expressions has been
   hardened to handle more edge cases during quote normalization (#2437)
 
+### _Blackd_
+
+- Replace sys.exit(-1) with raise ImportError (#2440)
+
 ### Integrations
 
 - The provided pre-commit hooks no longer specify `language_version` to avoid overriding

src/blackd/__init__.py

@@ -1,6 +1,5 @@
 import asyncio
 import logging
-import sys
 from concurrent.futures import Executor, ProcessPoolExecutor
 from datetime import datetime
 from functools import partial
@@ -11,13 +10,11 @@
     from aiohttp import web
     import aiohttp_cors
 except ImportError as ie:
-    print(
+    raise ImportError(
         f"aiohttp dependency is not installed: {ie}. "
         + "Please re-install black with the '[d]' extra install "
-        + "to obtain aiohttp_cors: `pip install black[d]`",
-        file=sys.stderr,
-    )
-    sys.exit(-1)
+        + "to obtain aiohttp_cors: `pip install black[d]`"
+    ) from None
 
 import black
 from black.concurrency import maybe_install_uvloop