xdrgen: Extend error reporting to AST transformation phase

xdrgen: Extend error reporting to AST transformation phase

Commit 277df18d7df9 ("xdrgen: Improve parse error reporting") added
clean, compiler-style error messages for syntax errors detected during
parsing. However, semantic errors discovered during AST transformation
still produce verbose Python stack traces.

When an XDR specification references an undefined type, the transformer
raises a VisitError wrapping a KeyError. Before this change:

  Traceback (most recent call last):
    File ".../lark/visitors.py", line 124, in _call_userfunc
      return f(children)
    ...
  KeyError: 'fsh4_mode'
  ...
  lark.exceptions.VisitError: Error trying to process rule "basic":
  'fsh4_mode'

After this change:

  file.x:156:2: semantic error
  Undefined type 'fsh4_mode'

      	fsh4_mode	mode;
              ^

The new handle_transform_error() function extracts position information
from the Lark tree node metadata and formats the error consistently with
parse error messages.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>