Issue 6477 - separate done and error callbacks in IFileSystem::Read

include/AdblockPlus/IFileSystem.h

Index: include/AdblockPlus/IFileSystem.h
diff --git a/include/AdblockPlus/IFileSystem.h b/include/AdblockPlus/IFileSystem.h
index 619b27139b6b1b744c2aaa2b11b44453d03f90e6..db69a410fb6dce8b465a4f302e4dfb35af90099c 100644
--- a/include/AdblockPlus/IFileSystem.h
+++ b/include/AdblockPlus/IFileSystem.h
@@ -69,18 +69,20 @@ namespace AdblockPlus
     /**
      * Callback type for the asynchronous Read call.
      * @param Output char array with file content.
-     * @param An error string. Empty if success.
      */
-    typedef std::function<void(IOBuffer&&,
-                               const std::string&)> ReadCallback;
+    typedef std::function<void(IOBuffer&&)> ReadCallback;
 
     /**
      * Reads from a file.
      * @param fileName File name.
-     * @param callback The function called on completion with the input data.
+     * @param doneCallback The function called on completion with the input
+     *   data. If this function throws then the implementation should call
+     *   `errorCallback`.
+     * @param errorCallback The function called if an error occured.
      */
     virtual void Read(const std::string& fileName,
-                      const ReadCallback& callback) const = 0;
+                      const ReadCallback& doneCallback,
+                      const Callback& errorCallback) const = 0;
 
     /**
      * Writes to a file.