Add error for missing file (#39)

Author: exussum12

src/DiffFileLoader.php

@@ -28,7 +28,7 @@ public function getChangedLines()
             !is_readable($this->fileLocation) &&
             strpos($this->fileLocation, "php://") !== 0
         )) {
-            throw new InvalidArgumentException("Can't read file");
+            throw new InvalidArgumentException("Can't read file", 1);
         }
 
         $handle = fopen($this->fileLocation, 'r');

tests/DiffFileLoadTest.php

@@ -22,6 +22,7 @@ public function testDiffResultsMatch($file, $expected)
     public function testNonExistantFile()
     {
         $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionCode(1);
         $this->getChangedLines('ufhbubfusdf');
     }