feat: friendly error messages for builder validation failures

[genro]

Summary

Builder validation errors (sub_tags, parent_tags) currently raise raw Python ValueErrors with cryptic messages and full tracebacks. They should raise a specific exception with a clear, actionable message.

Current behavior

When a builder call violates the schema (e.g. adding a child to an element that does not allow children), the error is:

ValueError: 'None' not allowed as child of 'tree'

With a multi-frame traceback through builder internals that is hard to interpret.

Proposed behavior

A dedicated exception class with a clear message:

BuilderValidationError: element 'tree' cannot contain children (sub_tags="")
  at: tabpane > tree
  hint: 'tree' is a leaf element — pass data via attributes, not as child nodes

Design

New exception class

class BuilderValidationError(Exception):
    """Raised when a builder call violates the schema."""

Error messages should include

• What failed: which element, which rule was violated
• Where: the parent > child path in the recipe
• Hint: actionable suggestion when possible

Cases to cover

1. Child tag not allowed: "element 'foo' is not allowed as child of 'bar' (allowed: baz, qux)"
2. Too many children of a type: "too many 'column' in 'datatable' (max: N)"
3. No children allowed: "element 'tree' cannot contain children (sub_tags='')"
4. Wrong parent: "element 'tabpane' must be inside 'tabbedcontent', found 'vertical'"

Files to modify

• src/genro_builders/builder.py — _validate_sub_tags, _validate_children_tags
• src/genro_builders/__init__.py — export BuilderValidationError