scanning: Create error slot for scan settings

This fixes a UI bug where the "Can't connect to scanners?" message
only shifts the Scanner dropdown and not also the Scanner label.

Before: http://screen/47MFUFFmwh4UMYm
After: http://screen/7Gm86RYfjAGWDV9

Bug: 1059779
Change-Id: I74bdd0cd50ff096dd0b54b9578526ae53c513691
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2588836Reviewed-by: Jesse Schettler <jschettler@chromium.org>
Commit-Queue: Gavin Williams <gavinwill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836682}

chromeos/components/scanning/resources/scan_settings_section.html

 <style include="scanning-shared scanning-fonts">
-  :host {
+  #selectDiv {
     align-items: center;
     display: flex;
     height: 48px;
@@ -31,6 +31,18 @@
     flex-shrink: 0;
     width: 192px;
   }
+
+  ::slotted([slot=error]) {
+    display: block;
+    margin-bottom: 8px;
+    margin-inline-end: 40px;
+    margin-inline-start: 120px;
+    margin-top: -8px;
+    width: 192px;
+  }
 </style>
-<slot name="label"></slot>
-<slot name="settings"></slot>
+<div id="selectDiv">
+  <slot name="label"></slot>
+  <slot name="settings"></slot>
+</div>
+<slot name="error"></slot>

chromeos/components/scanning/resources/scanner_select.html

@@ -2,7 +2,6 @@
   #scanHelp {
     color: var(--google-grey-700);
     font-size: 10px;
-    padding-top: 8px;
   }
 
   #scanHelpLink {
@@ -35,10 +34,11 @@
         </option>
       </template>
     </select>
-    <span id="scanHelp" hidden$="[[shouldHideScanHelpLink_(loaded, scanners)]]">
-      [[i18n('noScannersHelpText')]]
-      <!-- TODO(joonbug): Add href to the help link when available -->
-      <a id="scanHelpLink" href="#">[[i18n('noScannersHelpLinkLabel')]]</a>
-    </span>
+  </div>
+  <div id="scanHelp" slot="error"
+      hidden$="[[shouldHideScanHelpLink_(loaded, scanners)]]">
+    [[i18n('noScannersHelpText')]]
+    <!-- TODO(gavinwill): Add href to the help link when available -->
+    <a id="scanHelpLink" href="#">[[i18n('noScannersHelpLinkLabel')]]</a>
   </div>
 </scan-settings-section>