Error handling

[qTipTip]

We need better error handling.

I am envisioning something like this:

class PyletteError(Exception):
    """Base exception for Pylette"""
    def __init__(self, message: str, suggestion: str | None = None):
        self.suggestion = suggestion
        super().__init__(message)

class InvalidImageError(PyletteError):
    """Raised when image cannot be processed"""
    pass

...

whereby each error carries a suggestion-message.