Type tweaks for exceptions and some validation

lib/Exceptions/InvalidArtistException.php

@@ -0,0 +1,6 @@
+<?
+namespace Exceptions;
+
+class InvalidArtistException extends ValidationException{
+	protected $message = 'Artist is invalid.';
+}

lib/Exceptions/InvalidArtworkException.php

@@ -1,6 +1,6 @@
 <?
 namespace Exceptions;
 
-class InvalidArtworkException extends AppException{
+class InvalidArtworkException extends ValidationException{
 	protected $message = 'Artwork is invalid.';
 }

lib/Exceptions/InvalidArtworkTagException.php

@@ -1,5 +1,12 @@
 <?
 namespace Exceptions;
 
-class InvalidArtworkTagException extends AppException{
+class InvalidArtworkTagException extends ValidationException{
+	protected $message = 'Artwork tag is invalid.';
+
+	public function __construct(?string $tagName){
+		if($tagName !== null && trim($tagName) != ''){
+			$this->message = 'Artwork tag ' . $tagName . ' is invalid.';
+		}
+	}
 }

lib/Exceptions/InvalidNewsletterSubscription.php

@@ -0,0 +1,6 @@
+<?
+namespace Exceptions;
+
+class InvalidNewsletterSubscription extends ValidationException{
+	protected $message = 'Newsletter subscription is invalid.';
+}

lib/Exceptions/InvalidPollVoteException.php

@@ -0,0 +1,6 @@
+<?
+namespace Exceptions;
+
+class InvalidPollVoteException extends ValidationException{
+	protected $message = 'Vote is invalid.';
+}