Propagate Rust errors through as Java exceptions

• Error handling from Rust to Java is now greatly improved by properly utilizing Java checked and unchecked exceptions.
• We define a set of Java checked exception classes:
  • OnionmasqException: a general checked exception for all Rust errors with no more specific subclass
  • ArtiException: for Arti errors with no more specific subclass
  • ArtiStateException: when Arti failed to initialise due to a problem with its state or cache directories (which could be cleared from Java before retrying).
  • TorAccessException: when Arti failed to connect to the Tor network, likely due to local network conditions (which might want to be presented as some form of 'network down or censored' error?)
• Errors that cannot reasonably be expected to be handled in Rust, like panics, are now properly represented as unchecked exceptions in Java:
  • RustPanicException now represents panics, instead of the previously overly generic and wrong java.lang.Exception
  • JniMisuseException is raised for all JNI-related troubles.
• We might add more of these as needed to cover different failure cases; the translation is implemented by a new errors module in onionmasq-mobile.
  • We resist the temptation to define a custom error enum and just loop through the error chain at runtime.
  • This lets us keep the convenience and feature set of anyhow (no having to custom define enum variants, easy context addition, etc).
  • We can adjust the translation logic as needed in future easily, too!

cc @cyberta -- would you mind taking a look at the Java side of things? thanks!