change: Implement structured exception hierarchy

[mathysgrapotte]

Is your change request related to a problem? Please describe.

Current error handling uses generic exceptions without context, making debugging difficult in pipeline executions.

Describe the solution you'd like

1. Create stimulus/exceptions.py with base StimulusError and component-specific exceptions
2. Implement context-rich error formatting with component details
3. Update core classes to raise appropriate exception types

[mathysgrapotte]

example implementation for context :

from typing import Optional, Dict, Any

class StimulusError(Exception):
    """Base exception with context-aware formatting"""
    
    def __init__(self, msg: str, context: Optional[Dict[str, Any]] = None):
        super().__init__(msg)
        self.context = context or {}
        
    def add_context(self, **additional_context: Any) -> "StimulusError":
        """Chainable context builder"""
        self.context.update(additional_context)
        return self
        
    def __str__(self) -> str:
        base_msg = super().__str__()
        if self.context:
            ctx_str = " | ".join(f"{k}={v}" for k,v in self.context.items())
            return f"{base_msg} [{ctx_str}]"
        return base_msg

class LoaderConfigError(StimulusError):
    """Raised when loader configuration is invalid"""
    
# Usage in loader class
class DataLoader:
    def __init__(self, config: dict):
        if "columns" not in config:
            raise LoaderConfigError("Missing columns definition").add_context(
                config_keys=list(config.keys()),
                loader_type=self.__class__.__name__
            )