Merge branch '5.0' into 5.1

* 5.0:
  Fix abstract method name in PHP doc block
  Various cleanups
  [HttpClient] fix issues in tests
  Fixes sprintf(): Too few arguments in form transformer
  [Console] Fix QuestionHelper::disableStty()
  [Validator] Use Mime component to determine mime type for file validator
  validate subforms in all validation groups
  Update Hungarian translations
  Add meaningful message when Process is not installed (ProcessHelper)
  [PropertyAccess] Fix TypeError parsing again.
  [TwigBridge] fix fallback html-to-txt body converter
  [Security/Http] fix merge
  [ErrorHandler] fix setting $trace to null in FatalError
  [Form] add missing Czech validators translation
  [Validator] add missing Czech translations
  never directly validate Existence (Required/Optional) constraints

Author: nicolas-grekas

Filesystem.php

@@ -97,7 +97,7 @@ public function mkdir($dirs, int $mode = 0777)
                 if (!is_dir($dir)) {
                     // The directory was not created by a concurrent process. Let's throw an exception with a developer friendly error message if we have one
                     if (self::$lastError) {
-                        throw new IOException(sprintf('Failed to create "%s": '.self::$lastError, $dir), 0, null, $dir);
+                        throw new IOException(sprintf('Failed to create "%s": ', $dir).self::$lastError, 0, null, $dir);
                     }
                     throw new IOException(sprintf('Failed to create "%s".', $dir), 0, null, $dir);
                 }
@@ -167,16 +167,16 @@ public function remove($files)
             if (is_link($file)) {
                 // See https://bugs.php.net/52176
                 if (!(self::box('unlink', $file) || '\\' !== \DIRECTORY_SEPARATOR || self::box('rmdir', $file)) && file_exists($file)) {
-                    throw new IOException(sprintf('Failed to remove symlink "%s": '.self::$lastError, $file));
+                    throw new IOException(sprintf('Failed to remove symlink "%s": ', $file).self::$lastError);
                 }
             } elseif (is_dir($file)) {
                 $this->remove(new \FilesystemIterator($file, \FilesystemIterator::CURRENT_AS_PATHNAME | \FilesystemIterator::SKIP_DOTS));
 
                 if (!self::box('rmdir', $file) && file_exists($file)) {
-                    throw new IOException(sprintf('Failed to remove directory "%s": '.self::$lastError, $file));
+                    throw new IOException(sprintf('Failed to remove directory "%s": ', $file).self::$lastError);
                 }
             } elseif (!self::box('unlink', $file) && file_exists($file)) {
-                throw new IOException(sprintf('Failed to remove file "%s": '.self::$lastError, $file));
+                throw new IOException(sprintf('Failed to remove file "%s": ', $file).self::$lastError);
             }
         }
     }