Add alert for DN logins that dont exist. Might be attempts to use the rootdn which is not supported.

Closes #345

resources/views/errors/501.blade.php

@@ -0,0 +1,33 @@
+@extends('architect::layouts.error')
+
+@section('error')
+	501: @lang('LDAP Authentication Error')
+@endsection
+
+@section('content')
+	<table class="table table-sm table-borderless table-condensed">
+		<tr>
+			<th>@lang('Error')</th>
+		</tr>
+
+		<tr>
+			<td colspan="2">{{ $exception->getMessage() }}</td>
+		</tr>
+
+		<tr>
+			<th>@lang('Possible Causes')</th>
+		</tr>
+		<tr>
+			<td>
+				<ul class="ps-3">
+					<li>The DN you used to login actually doesnt exist in the server (DN's must exist in order to login)</li>
+					<li>You are attempting to use the <strong>rootdn</strong> to login (not supported)</li>
+				</ul>
+			</td>
+		</tr>
+	</table>
+
+	<p>To suppress this message, set <strong>LDAP_ALERT_ROOTDN</strong> to <strong>FALSE</strong> before starting PLA.</p>
+	<p>Back to <a href="{{ url('login') }}">login</a>?</p>
+
+@endsection