Connection dialog: help the operator fix a credential

Seen while checking AWS credential refusals in the Add/Edit connection dialog (Task 6724). Suggestions: 1. When the Credential field shows a refusal, put an "Edit credential" link next to it. It should open that credential's keys so the operator can fix them without leaving the dialog. Right now the only way out is Cancel, then Secrets. 2. After "Add new…" files a credential and AWS then refuses the connection, say once that the credential was saved and can be fixed or deleted. Otherwise a bad credential stays in the roster without the operator noticing. 3. The lookup warning and the save refusal name different EC2 actions (DescribeSecurityGroups vs DescribeVpcs) for the same bad keys. Keep the action name out of the plain lead, which already happens, and consider dropping it from the visible text entirely. Out of scope: how networks or images are resolved (apps/api/app/cloud_node_launch.py, apps/api/app/cloud_connection_network.py resolution logic). Done means: - A credential AWS refuses can be fixed from the dialog in one step, and an inline-filed credential that AWS refuses is called out once.

Connection dialog: "Edit credential" fixes refused AWS keys inline (Create PATCHes keys, then files the connection); an inline-filed refused credential is named once; EC2 action dropped from refusals. PR #5662 merged (cc276848c); deploy superseded. Follow-up draft 6789.