Simplify PrerequisiteInterface

I went with a return type of Collection, because it is easier to call
methods such as isEmpty() directly on those objects.

framework/core/views/install/app.php

@@ -129,30 +129,30 @@
         animation-name: fadeIn;
       }
 
-      .Errors {
+      .Problems {
         margin-top: 50px;
       }
-      .Errors .Error:first-child {
+      .Problems .Problem:first-child {
         border-top-left-radius: 4px;
         border-top-right-radius: 4px;
       }
-      .Errors .Error:last-child {
+      .Problems .Problem:last-child {
         border-bottom-left-radius: 4px;
         border-bottom-right-radius: 4px;
       }
-      .Error {
+      .Problem {
         background: #EDF2F7;
         margin: 0 0 1px;
         padding: 20px 25px;
         text-align: left;
       }
-      .Error-message {
+      .Problem-message {
         font-size: 16px;
         color: #3C5675;
         font-weight: normal;
         margin: 0;
       }
-      .Error-detail {
+      .Problem-detail {
         font-size: 13px;
         margin: 5px 0 0;
       }

framework/core/views/install/errors.php

@@ -1,14 +0,0 @@
-<h2>Hold Up!</h2>
-
-<p>These errors must be resolved before you can continue the installation. If you're having trouble, get help on the <a href="https://flarum.org/docs/install.html" target="_blank">Flarum website</a>.</p>
-
-<div class="Errors">
-  <?php foreach ($errors as $error): ?>
-    <div class="Error">
-      <h3 class="Error-message"><?php echo $error['message']; ?></h3>
-      <?php if (! empty($error['detail'])): ?>
-        <p class="Error-detail"><?php echo $error['detail']; ?></p>
-      <?php endif; ?>
-    </div>
-  <?php endforeach; ?>
-</div>

framework/core/views/install/problems.php

@@ -0,0 +1,14 @@
+<h2>Hold Up!</h2>
+
+<p>These problems must be resolved before you can continue the installation. If you're having trouble, get help on the <a href="https://flarum.org/docs/install.html" target="_blank">Flarum website</a>.</p>
+
+<div class="Problems">
+  <?php foreach ($problems as $problem): ?>
+    <div class="Problem">
+      <h3 class="Problem-message"><?php echo $problem['message']; ?></h3>
+      <?php if (! empty($problem['detail'])): ?>
+        <p class="Problem-detail"><?php echo $problem['detail']; ?></p>
+      <?php endif; ?>
+    </div>
+  <?php endforeach; ?>
+</div>