client: handle 422 errors on the client

When the client encounters a 422 HTTP error, we now raise a
RecoverableClientError. Client controllers can catch the error and print
a dialog with the error message in the UI.

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

subiquity/client/client.py

@@ -16,6 +16,7 @@
 import asyncio
 import contextvars
 import inspect
+import json
 import logging
 import os
 import signal
@@ -59,6 +60,10 @@ class Abort(Exception):
         self.error_report_ref = error_report_ref
 
 
+class RecoverableClientError(ValueError):
+    pass
+
+
 DEBUG_SHELL_INTRO = _(
     """\
 Installer shell session activated.
@@ -206,7 +211,10 @@ class SubiquityClient(TuiApplication):
             raise Abort(ref)
         try:
             response.raise_for_status()
-        except aiohttp.ClientError:
+        except aiohttp.ClientError as exc:
+            if isinstance(exc, aiohttp.ClientResponseError) and exc.status == 422:
+                raise RecoverableClientError(json.loads(headers["x-error-msg"]))
+
             report = self.error_reporter.make_apport_report(
                 ErrorReportKind.SERVER_REQUEST_FAIL,
                 "request to {}".format(response.url.path),