Add classes and tests from common project

src/DocumentException.php

@@ -0,0 +1,30 @@
+<?php declare(strict_types=1);
+
+namespace BitBadger\PDODocument;
+
+use Exception;
+use Throwable;
+
+/**
+ * Exceptions occurring during document processing
+ */
+class DocumentException extends Exception
+{
+    /**
+     * Constructor
+     *
+     * @param string $message A message for the exception
+     * @param int $code The code for the exception (optional; defaults to 0)
+     * @param Throwable|null $previous The previous exception (optional; defaults to `null`)
+     */
+    public function __construct(string $message, int $code = 0, ?Throwable $previous = null)
+    {
+        parent::__construct($message, $code, $previous);
+    }
+
+    public function __toString(): string
+    {
+        $codeStr = $this->code == 0 ? '' : "[$this->code] ";
+        return __CLASS__ . ": $codeStr$this->message\n";
+    }
+}