[Fix] [forecasting] error handling is not clear when a error is raised with regressor

[lisphilar]

Summary

Error handling is not clear when some errors is raised with regressor. When a internal ValueError occurs, the error description is always ValueError: max() arg is an empty sequence. This is related to #778.

Codes

import covsirphy as cs
# Dataset preparation
data_loader = cs.DataLoader("input")
jhu_data = data_loader.jhu()
oxcgr_data = data_loader.oxcgrt()
# Scenario analysis
snl = cs.Scenario(country="Italy")
snl.register(jhu_data, extras=[oxcgrt_data]))
snl.trend()
snl.estimate(cs.SIRF)
snl.fit()

Outputs

If some ValueError occured with regressors, ValueError: max() arg is an empty sequence.

Environment

• CovsirPhy version: 2.20.1
• Python version: 3.9.5
• Installation: poetry
• System: WSL (Ubuntu)

[lisphilar]

When all regressors failed in select delay value (i.e. some parameter values are out of (0, 1)), UnExpectedReturnValueError will be returned.