Corrupted recovery file will stop workbench from opening

[jhaigh0]

When updating the project recovery code (#36859) I changed the find_number_of_workspaces_in_directory method to read in the .recfile which is a json dump of things like the open plots.

If this file is corrupted somehow (maybe mantid is closed / crashses while the json write is happening?) there will be an uncaught exception when opening workbench.

To Reproduce
I somehow got this to happen, I thought it was related to #37168 but I think in that cases no file is written.
The best thing would be to open workbench and open a few interfaces and plots, then wait for recovery to run.
Then go and find the .recfile (on windows recovery data is in
C\Users\<user_name>\AppData\Roaming\mantidproject\mantid\workbench-recovery) and delete something from the end of the json string.

Expected behavior
The checkpoint should fail and not be an option in the checkpoint selection widget.

Platform/Version (please complete the following information):

• OS: Windows
• OS Version:
• Mantid Version Main