fixup scope of try/except around forward/backwards analysis

[richardjgowers]

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[pep8speaks]

Hello @richardjgowers! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file openfe/tests/protocols/conftest.py:

Line 241:80: E501 line too long (92 > 79 characters)
Line 242:80: E501 line too long (86 > 79 characters)
Line 243:80: E501 line too long (96 > 79 characters)
Line 244:80: E501 line too long (92 > 79 characters)
Line 245:80: E501 line too long (113 > 79 characters)

• In the file openfe/tests/protocols/test_openmmutils.py:

Line 510:80: E501 line too long (115 > 79 characters)