Describe .archify (YAML) as the diagram format everywhere (0.5.1)

- Marketplace description, README and messages lead with .archify files
  in YAML; Archify's JSON files and rendered HTML are described as also
  supported, in their own README section and a supported-files table
- Rename the rendered-HTML editor to Archify HTML Viewer (it shared the
  extension's name) and its command to Open in Archify HTML Viewer
- Empty or broken .archify files ask for valid YAML instead of JSON

package.json

@@ -1,8 +1,8 @@
 {
   "name": "archify-vscode-ext",
   "displayName": "Archify Diagram Viewer",
-  "description": "Visualize Archify diagrams: .archify files (YAML) open straight into the interactive diagram; also previews Archify JSON sources and rendered Archify HTML inside VS Code.",
-  "version": "0.5.0",
+  "description": "Archify diagrams in VS Code. Write diagrams as .archify files in YAML: they open as the interactive diagram, with live updates, validation, templates and completion. Also previews Archify's own JSON files (*.architecture.json, \u2026) and opens rendered Archify HTML.",
+  "version": "0.5.1",
   "publisher": "root-at-skic",
   "license": "MIT",
   "icon": "media/icon.png",
@@ -23,6 +23,7 @@
   ],
   "keywords": [
     "archify",
+    "yaml",
     "diagram",
     "architecture",
     "sequence",
@@ -57,7 +58,7 @@
       },
       {
         "command": "archify.openHtmlViewer",
-        "title": "Open in Archify Viewer",
+        "title": "Open in Archify HTML Viewer",
         "category": "Archify",
         "icon": "$(open-preview)"
       },
@@ -212,7 +213,7 @@
       },
       {
         "viewType": "archify.htmlViewer",
-        "displayName": "Archify Diagram Viewer",
+        "displayName": "Archify HTML Viewer",
         "selector": [
           {
             "filenamePattern": "*.html"