Move errors to core/error

[ba6ul]

Supabase exceptions in core/error

class SupabaseAuthException extends NetworkException {
const SupabaseAuthException([String message = 'Authentication failed.'])
: super(message);
}

class SupabaseDatabaseException extends NetworkException {
const SupabaseDatabaseException([String message = 'Database operation failed.'])
: super(message);
}

class SupabaseStorageException extends NetworkException {
const SupabaseStorageException([String message = 'Storage operation failed.'])
: super(message);
}