More informative error message when Symbol encoding is invalid
When a symbol literal is invalid in the source file's encoding, an
EncodingError exception is raised, with the following message:
invalid encoding symbol. This patches the interpreter so a more informative message is generated, similar to:
encoding UTF-8 is invalid for symbol :"\xC7".
Here is where this originated from: https://github.com/whitequark/parser/issues/252