Stop bundling errorWrapper CSS rules in the MOZCENTRAL build

Note that both the `errorWrapper` HTML and JavaScript code is being ignored in the MOZCENTRAL build, i.e. the *built-in* Firefox PDF Viewer, however the CSS rules are still being included.
That seems totally unnecessary, and while we currently don't have full build-target support in the CSS pre-processor we can actually improve things quite easily anyway. By (ab)using the existing CSS pre-processor, which will remove any non-Firefox CSS rules for the MOZCENTRAL build, it's possible to easily stop bundling any CSS rules by using comments that include a `-webkit`-string.

*Please note:* To easily test that this doesn't break the `errorWrapper` in GENERIC builds, try running e.g. `PDFViewerApplication._otherError("test");` in the web-console.

web/viewer.css

@@ -29,7 +29,7 @@
 
   --main-color: rgba(12, 12, 13, 1);
   --body-bg-color: rgba(237, 237, 240, 1);
-  --errorWrapper-bg-color: rgba(255, 110, 110, 1);
+  --errorWrapper-bg-color: rgba(255, 110, 110, 1); /* -webkit-non-mozcentral */
   --progressBar-color: rgba(10, 132, 255, 1);
   --progressBar-indeterminate-bg-color: rgba(221, 221, 222, 1);
   --progressBar-indeterminate-blend-color: rgba(116, 177, 239, 1);
@@ -112,7 +112,7 @@
   :root {
     --main-color: rgba(249, 249, 250, 1);
     --body-bg-color: rgba(42, 42, 46, 1);
-    --errorWrapper-bg-color: rgba(169, 14, 14, 1);
+    --errorWrapper-bg-color: rgba(169, 14, 14, 1); /* -webkit-non-mozcentral */
     --progressBar-color: rgba(0, 96, 223, 1);
     --progressBar-indeterminate-bg-color: rgba(40, 40, 43, 1);
     --progressBar-indeterminate-blend-color: rgba(20, 68, 133, 1);
@@ -1194,7 +1194,7 @@ a:focus > .thumbnail > .thumbnailSelectionRing,
   background: rgba(0, 0, 255, 0.3);
 }
 
-#errorWrapper {
+#errorWrapper /* -webkit-non-mozcentral */ {
   background-color: var(--errorWrapper-bg-color);
   color: var(--main-color);
   left: 0;
@@ -1204,14 +1204,17 @@ a:focus > .thumbnail > .thumbnailSelectionRing,
   padding: 3px 6px;
 }
 
-#errorMessageLeft {
+#errorMessageLeft /* -webkit-non-mozcentral */ {
   float: left;
 }
-#errorMessageRight {
+#errorMessageRight /* -webkit-non-mozcentral */ {
   float: right;
 }
 
-#errorMoreInfo {
+#errorSpacer /* -webkit-non-mozcentral */ {
+  clear: both;
+}
+#errorMoreInfo /* -webkit-non-mozcentral */ {
   background-color: var(--field-bg-color);
   color: var(--field-color);
   border: 1px solid var(--field-border-color);
@@ -1300,10 +1303,6 @@ dialog :link {
   margin-top: 10px;
 }
 
-.clearBoth {
-  clear: both;
-}
-
 .grab-to-pan-grab {
   cursor: grab !important;
 }

web/viewer.html

@@ -344,7 +344,7 @@ See https://github.com/adobe-type-tools/cmap-resources
               Close
             </button>
           </div>
-          <div class="clearBoth"></div>
+          <div id="errorSpacer"></div>
           <textarea id="errorMoreInfo" hidden='true' readonly="readonly"></textarea>
         </div>
 <!--#endif-->