Provide relevant error message if datadir is not writable.

src/util.cpp

@@ -418,6 +418,19 @@ void ReleaseDirectoryLocks()
     dir_locks.clear();
 }
 
+bool DirIsWritable(const fs::path& directory)
+{
+    fs::path tmpFile = directory / fs::unique_path();
+
+    FILE* file = fsbridge::fopen(tmpFile, "a");
+    if (!file) return false;
+
+    fclose(file);
+    remove(tmpFile);
+
+    return true;
+}
+
 /** Interpret string as boolean, for argument parsing */
 static bool InterpretBool(const std::string& strValue)
 {