Use sys.exit instead of exit

[valefar-on-discord]

What I did
Replaced usages of exit with sys.exit as was the case in deposit.py

Related issue
FIxes #175

[remyroy]

Everything is good on my side.

[github-actions]

Test Coverage: Download HTML Report

Name                                                                 Stmts   Miss  Cover
----------------------------------------------------------------------------------------
ethstaker_deposit/__init__.py                                            1      0   100%
ethstaker_deposit/bls_to_execution_change_keystore.py                   33      2    94%
ethstaker_deposit/cli/__init__.py                                        0      0   100%
ethstaker_deposit/cli/existing_mnemonic.py                              42      0   100%
ethstaker_deposit/cli/exit_transaction_keystore.py                      44      3    93%
ethstaker_deposit/cli/exit_transaction_mnemonic.py                      65      9    86%
ethstaker_deposit/cli/generate_bls_to_execution_change.py               62     11    82%
ethstaker_deposit/cli/generate_bls_to_execution_change_keystore.py      45      3    93%
ethstaker_deposit/cli/generate_keys.py                                  44      2    95%
ethstaker_deposit/cli/new_mnemonic.py                                   32      0   100%
ethstaker_deposit/cli/partial_deposit.py                                65      5    92%
ethstaker_deposit/cli/test_keystore.py                                  23      1    96%
ethstaker_deposit/credentials.py                                       197     68    65%
ethstaker_deposit/deposit.py                                            58     10    83%
ethstaker_deposit/exceptions.py                                          7      0   100%
ethstaker_deposit/key_handling/__init__.py                               0      0   100%
ethstaker_deposit/key_handling/key_derivation/__init__.py                0      0   100%
ethstaker_deposit/key_handling/key_derivation/mnemonic.py               95      6    94%
ethstaker_deposit/key_handling/key_derivation/path.py                   17      1    94%
ethstaker_deposit/key_handling/key_derivation/tree.py                   36      0   100%
ethstaker_deposit/key_handling/keystore.py                             100      0   100%
ethstaker_deposit/settings.py                                           26      0   100%
ethstaker_deposit/utils/__init__.py                                      0      0   100%
ethstaker_deposit/utils/ascii_art.py                                     2      0   100%
ethstaker_deposit/utils/click.py                                        72      3    96%
ethstaker_deposit/utils/config.py                                        3      0   100%
ethstaker_deposit/utils/constants.py                                    29      0   100%
ethstaker_deposit/utils/crypto.py                                       29      1    97%
ethstaker_deposit/utils/deposit.py                                       9      0   100%
ethstaker_deposit/utils/exit_transaction.py                             24      0   100%
ethstaker_deposit/utils/file_handling.py                                 8      0   100%
ethstaker_deposit/utils/intl.py                                         54      3    94%
ethstaker_deposit/utils/ssz.py                                          60      7    88%
ethstaker_deposit/utils/terminal.py                                     28     20    29%
ethstaker_deposit/utils/validation.py                                  244     52    79%
----------------------------------------------------------------------------------------
TOTAL                                                                 1554    207    87%