Improve affix_stack error on Affix file / class name mismatch

[choksi81]

If the file containing an Affix component class is not named class name (in lower case) + .r2py, then calling affix_stack.find_and_register_affix might succeed importing, but fail registering internally the component, and throw a rather unspecific KeyError.

In the code leading to the traceback below,

• transparenttcprelayaffix.r2py defines a class Transparent_TCPRelayAffix;
• the user code tries to instantiate an Affix stack including TransparentTCPRelayAffix (which yields the filename when .lowered, but is not the class name), and
• when affix_stack.find_and_register_affix tries to copy the Affix's _context, that context (named after the class) doesn't exist, resulting in the KeyError.

It would be a good idea to check for this mismatch, and raise a more descriptive error message. (I assume a developer running unit tests would catch it very early on in the development, and trivially correct this mistake).

Uncaught exception!

---
Following is a full traceback, and a user traceback.
The user traceback excludes non-user modules. The most recent call is displayed last.

Full debugging traceback:
  "repy.py", line 154, in execute_namespace_until_completion
  "/private/tmp/affix/RUNNABLE/virtual_namespace.py", line 117, in evaluate
  "/private/tmp/affix/RUNNABLE/safe.py", line 588, in safe_run
  "dylink.r2py", line 546, in <module>
  "dylink.r2py", line 407, in dylink_dispatch
  "dylink.r2py", line 520, in evaluate
  "/private/tmp/affix/RUNNABLE/virtual_namespace.py", line 117, in evaluate
  "/private/tmp/affix/RUNNABLE/safe.py", line 588, in safe_run
  "test-ttr.r2py", line 5, in <module>
  "affix_stack.r2py", line 32, in __init__
  "affix_stack.r2py", line 133, in build_stack
  "affix_stack.r2py", line 218, in find_and_register_affix
  "/private/tmp/affix/RUNNABLE/safe.py", line 678, in __getitem__

User traceback:
  "dylink.r2py", line 546, in <module>
  "dylink.r2py", line 407, in dylink_dispatch
  "dylink.r2py", line 520, in evaluate
  "test-ttr.r2py", line 5, in <module>
  "affix_stack.r2py", line 32, in __init__
  "affix_stack.r2py", line 133, in build_stack
  "affix_stack.r2py", line 218, in find_and_register_affix

Exception (with type 'exceptions.KeyError'): 'Transparent_TCPRelayAffix'