Add DatasetNotFoundError for consistent error messages

[adamjstewart]

Summary

We should add a torchgeo.datasets.utils.DatasetNotFoundError class for consistent error messages when datasets aren't found.

Rationale

There is currently very little consistency between the errors reported by different datasets. Some raise RuntimeError, others raise FileNotFoundError. There are dozens of different messages raised by each dataset with little commonality between the text. Some mention using download=True, others mention changing the root directory. Some print the root directory, others leave users guessing.

Implementation

We should subclass FileNotFoundError and except the dataset instance as input. With this, we can check for download and checksum attributes to determine the appropriate message. Some possible formats include:

# Has a `self.download` attribute
f"Dataset not found in `root={self.root}` and `download=False`, either specify a different `root` directory or use `download=True` to automatically download the dataset."

# Doesn't have a `self.download` attribute
f"Dataset not found in `root={self.root}`, either specify a different `root` directory or manually download the dataset."

This will also require consistent documentation in docstrings. Maybe something like:

# Has a ``self.download`` attribute
Raises:                                                                          
    FileNotFoundError: if ``download=False`` but dataset is not found            
        or checksums don't match          

# Doesn't have a ``self.download`` attribute
Raises:                                                                          
    FileNotFoundError: if dataset is not found   

Of course, we'll also need to update the tests as well.

Alternatives

We could simply update everything to have common text, but things would rapidly become out of sync without a centralized class to standardize the error message. This also avoids code duplication.

Additional information

No response