Updated not-found image path handling to have better ux

Added test to cover.
Started refactoring some of the app error handling in
the process of this.

Fixes #2696

app/Exceptions/PrettyException.php

@@ -1,6 +1,44 @@
 <?php namespace BookStack\Exceptions;
 
-class PrettyException extends \Exception
-{
+use Exception;
+use Illuminate\Contracts\Support\Responsable;
+use Illuminate\Http\Request;
 
+class PrettyException extends Exception implements Responsable
+{
+    /**
+     * @var ?string
+     */
+    protected $subtitle = null;
+
+    /**
+     * @var ?string
+     */
+    protected $details = null;
+
+    /**
+     * Render a response for when this exception occurs.
+     * @param Request $request
+     */
+    public function toResponse($request)
+    {
+        $code = ($this->getCode() === 0) ? 500 : $this->getCode();
+        return response()->view('errors.' . $code, [
+            'message' => $this->getMessage(),
+            'subtitle' => $this->subtitle,
+            'details' => $this->details,
+        ], $code);
+    }
+
+    public function setSubtitle(string $subtitle): self
+    {
+        $this->subtitle = $subtitle;
+        return $this;
+    }
+
+    public function setDetails(string $details): self
+    {
+        $this->details = $details;
+        return $this;
+    }
 }