Fix grammatical error, and make wording consistent (#410)

- "File can't be open" should be "File can't be opened"
- Use cannot instead of "can not" or "can't" for consistency

src/ModelManager.php

@@ -12,29 +12,29 @@ class ModelManager
     public function saveToFile(Estimator $estimator, string $filepath): void
     {
         if (!is_writable(dirname($filepath))) {
-            throw new FileException(sprintf('File "%s" can\'t be saved.', basename($filepath)));
+            throw new FileException(sprintf('File "%s" cannot be saved.', basename($filepath)));
         }
 
         $serialized = serialize($estimator);
         if (!isset($serialized[0])) {
-            throw new SerializeException(sprintf('Class "%s" can not be serialized.', gettype($estimator)));
+            throw new SerializeException(sprintf('Class "%s" cannot be serialized.', gettype($estimator)));
         }
 
         $result = file_put_contents($filepath, $serialized, LOCK_EX);
         if ($result === false) {
-            throw new FileException(sprintf('File "%s" can\'t be saved.', basename($filepath)));
+            throw new FileException(sprintf('File "%s" cannot be saved.', basename($filepath)));
         }
     }
 
     public function restoreFromFile(string $filepath): Estimator
     {
         if (!file_exists($filepath) || !is_readable($filepath)) {
-            throw new FileException(sprintf('File "%s" can\'t be open.', basename($filepath)));
+            throw new FileException(sprintf('File "%s" cannot be opened.', basename($filepath)));
         }
 
         $object = unserialize((string) file_get_contents($filepath), [Estimator::class]);
         if ($object === false) {
-            throw new SerializeException(sprintf('"%s" can not be unserialized.', basename($filepath)));
+            throw new SerializeException(sprintf('"%s" cannot be unserialized.', basename($filepath)));
         }
 
         return $object;