Fix failing file mode validation tests

See http-interop/http-factory-tests#42

src/StreamFactory.php

@@ -42,6 +42,9 @@
 use Psr\Http\Message\StreamFactoryInterface;
 use Psr\Http\Message\StreamInterface;
 
+use InvalidArgumentException;
+use RuntimeException;
+
 class StreamFactory implements StreamFactoryInterface
 {
     /**
@@ -65,11 +68,23 @@ public function createStream(string $content = ""): StreamInterface
      */
     public function createStreamFromFile(string $filename, string $mode = "r"): StreamInterface
     {
+        if ("" === $mode || false === in_array($mode[0], ["r", "w", "a", "x", "c"])) {
+            throw new InvalidArgumentException(
+                sprintf("The mode %s is invalid.", $mode)
+            );
+        }
+
         if (class_exists(SlimPsr7StreamFactory::class)) {
             return (new SlimPsr7StreamFactory)->createStreamFromFile($filename, $mode);
         }
 
-        $resource = fopen($filename, $mode);
+        $resource = @fopen($filename, $mode);
+        if (false === $resource) {
+            throw new RuntimeException(
+                sprintf("The file %s cannot be opened.", $filename)
+            );
+        }
+
         return $this->createStreamFromResource($resource);
     }
 
@@ -98,6 +113,6 @@ public function createStreamFromResource($resource): StreamInterface
             return new GuzzleStream($resource);
         }
 
-        throw new \RuntimeException("No PSR-7 implementation available");
+        throw new RuntimeException("No PSR-7 implementation available");
     }
 }