Allow loading a custom threats file

[nineinchnick]

Allow loading a custom threats file by setting a new TM.threatsFile attribute.

Added throwing an exception when an attribute is already set, instead of silently ignoring the new value, which could lead to surprising errors.

Added missing unit tests.

[ghost]

DeepCode's analysis on #2583a4 found:

• ❌ 0 critical issues. ⚠️ 0 warnings and 1 minor issue. ✔️ 1 issue were fixed.

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[izar]

This is great, thank you so much (incidentally do you have a Twitter handle i can add when I post about the latest additions you made?) !
I am wondering if for completeness the name of the threat file added should be part of the report.

[nineinchnick]

My Twitter handle is the same as my username here: https://twitter.com/n1neinchnick

Adding threats filename might be a good idea but on the other hand it does already copy selected threat properties. The default template could include threat IDs. The file itself can change after generating the report so I'm not sure how useful that would be, other than just saying a custom one was used instead of the default.

BTW I got more PRs queued up, just need to get them in one by one to avoid conflicts.