Improve error message when trying to conditionally sample before fitting

[srinify]

Problem Description

If a user tries to conditionally sample before fitting the model, the error message surface to the user isn't very helpful.

While we improved the error message when a user does this with regular sampling, we didn't for when they try to conditionally sample.

Current error message:

# Scenario 1 (output_file_path not provided)
NotFittedError: Error: Sampling terminated. No results were saved due to unspecified "output_file_path".

# Scenario 2 (output_file_path provided)
sdv.data_processing.errors.NotFittedError: Error: Sampling terminated. Partial results are stored in C:\User\GaussianCopulaSample.csv.

Code to Reproduce

from sdv.datasets.demo import download_demo
from sdv.single_table import GaussianCopulaSynthesizer
from sdv.sampling import Condition

data, metadata = download_demo(
    modality='single_table',
    dataset_name='fake_hotel_guests'
)

synthesizer = GaussianCopulaSynthesizer(metadata)
condition0 = Condition(num_rows=454, column_values={"room_type":"BASIC"})
condition1 = Condition(num_rows=455, column_values={"room_type": "DELUXE"})

synthesizer.sample_from_conditions(max_tries_per_batch=100000, batch_size=1000, conditions=[condition0, condition1])

Expected Behavior

Instead of attempting to sample, the single table synthesizer should check first whether the synthesizer has been fitted. If it has not been fitted, we should proactively show a SamplingError explaining to the user what they must do.

SamplingError: This synthesizer has not been fitted. Please fit your synthesizer first before 
conditionally sampling synthetic data.