Custom errors should inherit from `StandardError` (not `Exception`)

[Petr Pučil]

See https://www.honeybadger.io/blog/ruby-exception-vs-standarderror-whats-the-difference/#custom-exceptions-should-inherit-from-standarderror:

It means you should always inherit from StandardError, and NEVER from Exception. Inheriting from Exception is bad because it breaks the expected behavior of rescue. People will think they're rescuing all application-level errors but yours will just sail on through.

kaitai_struct_ruby_runtime/lib/kaitai/struct/struct.rb

Lines 688 to 692 in ecf6751

 ## 
 # Common ancestor for all error originating from Kaitai Struct usage. 
 # Stores KSY source path, pointing to an element supposedly guilty of 
 # an error. 
 class KaitaiStructError < Exception 

But UnexpectedDataError should be left as is, because it only remains for compatibility with 0.8 and older KS versions (and will be eventually removed):

kaitai_struct_ruby_runtime/lib/kaitai/struct/struct.rb

Lines 78 to 84 in ecf6751

 ## 
 # Unused since Kaitai Struct Compiler v0.9+ - compatibility with 
 # older versions. 
 # 
 # Exception class for an error that occurs when some fixed content 
 # was expected to appear, but actual data read was different. 
 class UnexpectedDataError < Exception 

[Petr Pučil]

Yeah, this issue is also reported by RuboCop:

lib/kaitai/struct/struct.rb:692:27: W: [Correctable] Lint/InheritException: Inherit from StandardError instead of Exception.
class KaitaiStructError < Exception
                          ^^^^^^^^^

We should adopt kaitai-io/kaitai_struct_visualizer#38 for runtime library too, and make sure that RuboCop at least doesn't issue any warnings.