catch multiple exceptions for np.polyfit

[anilbey]

Description

Catches both errors (np.linalg.LinAlgError, SystemError) in np.polyfit call.

Checklist:

• Unit tests are added to cover the changes (skip if not applicable).
• The changes are mentioned in the documentation (skip if not applicable).
• CHANGELOG file is updated (skip if not applicable).

[darshanmandge]

For future reader: this was needed for ISI_log_slope.
Works now. Thanks.