Connection dialog: plain-language AWS refusals

Seen while checking the Add/Edit connection dialog (Task 6709) with a credential AWS refuses. Suggestions: 1. The Credential field's save refusal reads "AWS refused DescribeVpcs: AuthFailure — …": lead with a plain sentence ("AWS rejected this credential's keys") and keep AWS's own text after it, so a reader need not know EC2 action names. 2. The save refusal under Credential ends without a full stop while the lookup warning closes AWS's sentence ("… credentials."); close both the same way. 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 shows a plain-language first sentence beside the Credential field, AWS's text kept after it, punctuated the same as the lookup warning.

Connection dialog: when AWS refuses a credential, the message now starts with a plain sentence (e.g. "AWS rejected this credential’s keys."), keeps AWS's text, and ends with a full stop like the lookup warning. Quick gate green. Visual check done against a live AWS AuthFailure. PR #5648 merged (22b3952); its deploy was cancelled by a newer one. Follow-up draft: #6770.