Rework exception handling in tool API (as part of ceylon/ceylon-compiler#1170), removing @NonFatal and adding ToolError and FatalToolError for tool-specific exceptions to subclass.
Make constructors for ToolException (used for exception cases in the tool API) package vis, annoyingly except one which is used in the compiler tests
Remove some unused methods, since the hack used for ceylon/ceylon-runtime#20 is no longer needed.