Refactor error handling. Add 404 page

Website/Views/Error/PageNotFound.cshtml

@@ -0,0 +1,7 @@
+@model ErrorViewModel
+@{
+	ViewData["Title"] = "404 Not Found";
+}
+
+<h1>404 Not Found - page unavailable.</h1>
+This page cannot be found.

Website/Views/Error/ServerError.cshtml

@@ -0,0 +1,25 @@
+@model ErrorViewModel
+@{
+	ViewData["Title"] = "Error";
+}
+
+<h1 class="text-danger">Error.</h1>
+<h2 class="text-danger">An error occurred while processing your request.</h2>
+
+@if (Model.ShowRequestId)
+{
+	<p>
+		<strong>Request ID:</strong> <code>@Model.RequestId</code>
+	</p>
+}
+
+<h3>Development Mode</h3>
+<p>
+	Swapping to <strong>Development</strong> environment will display more detailed information about the error that occurred.
+</p>
+<p>
+	<strong>The Development environment shouldn't be enabled for deployed applications.</strong>
+	It can result in displaying sensitive information from exceptions to end users.
+	For local debugging, enable the <strong>Development</strong> environment by setting the <strong>ASPNETCORE_ENVIRONMENT</strong> environment variable to <strong>Development</strong>
+	and restarting the app.
+</p>