Add a parent error class

[n00tmeg]

Adding a parent error class could be useful when we want to rescue all the errors related to ruby_smb. For example:

begin
  tree = client.tree_connect(path)
  ...[SNIP]...
rescue RubySMBError => e
  puts "An error occurred: #{e.message}"
end

[dmohanty-r7]

Fixed by #108