Improve unit test coverage for accession views

[helrond]

Is your feature request related to a problem? Please describe.

The accession views file is not fully tested.

Describe the solution you'd like

Add additional tests or tweak existing tests to improve coverage of bag_transfer/accession/views.py

Additional context

Current coverage is missing the following lines:

• 36
• 59-84
• 114-115
• 138-140
• 147-152
• 166
• 199
• 202-214
• 221
• 223
• 226
• 272-287
• 311
• 315
• 321-323
• 326-329
• 338-341