Revisions: Better error handling.

* Shows an error message if the current diff can't be loaded.
* For bulk pre-loading, catches errors, and cuts subsequent requests in half, until eventually giving up.
* Some CSS fixes related to this, and the loading spinner.
* `wp.revisions.loadAll()` now returns a promise representing whether or not all revisions could be loaded.

Fixes #24758.

git-svn-id: https://develop.svn.wordpress.org/trunk@24732 602fd350-edb4-49c9-b593-d223f7449a82

wp-admin/css/wp-admin.css

@@ -3554,16 +3554,24 @@ body.revision-php {
 	height: 140px;
 }
 
-.revisions .loading-indicator {
+.revisions .diff-error {
 	position: absolute;
 	text-align: center;
-	vertical-align: middle;
-	opacity: 0;
 	margin: 0 auto;
 	width: 100%;
-	height: 32px;
+	display: none;
+}
+
+.revisions.diff-error .diff-error {
+	display: block;
+}
+
+.revisions .loading-indicator {
+	position: absolute;
+	vertical-align: middle;
+	opacity: 0;
+	width: 100%;
 	top: 3em;
-	background: #fff url(../images/wpspin_light-2x.gif) no-repeat center top;
 	-webkit-transition: opacity 0.5s;
 	-moz-transition:    opacity 0.5s;
 	-ms-transition:     opacity 0.5s;
@@ -3571,6 +3579,12 @@ body.revision-php {
 	transition:         opacity 0.5s;
 }
 
+.revisions .loading-indicator span.spinner {
+	display: block;
+	margin: 0 auto;
+	float: none;
+}
+
 .revisions.loading .loading-indicator {
 	opacity: 1;
 }
@@ -3587,6 +3601,10 @@ body.revision-php {
 	opacity: 0.5;
 }
 
+.revisions.diff-error .diff {
+	visibility: hidden;
+}
+
 .revisions-meta {
 	margin-top: 15px;
 }