sts assumes ~/.cache directory exists

[kfiresmith]

Hi Folks,
When attempting to run sts as a newly created system user with a non-skeletonized home directory, I discovered that sts assumes that ~/.cache already exists and is available to create sts-client-id.txt within.

Here's what I observed:

$ /opt/sts/bin/sts 
panic: open /opt/<service-account-name>/.cache/sts-client-id.txt: no such file or directory

goroutine 1 [running]:
main.runFromServer({0xc69a20, 0x7b})
        /dap/apps/godev/src/sts/main/controlled.go:166 +0xd14
main.main()
        /dap/apps/godev/src/sts/main/app.go:50 +0x2c

$ mkdir /opt/<service-account-name>/.cache

$ /opt/sts/bin/sts 
2024-02-14T21:10:38Z [Server: a2e.energy.gov:443]
2024-02-14T21:10:39Z [Server Prefix: /_upload]
2024-02-14T21:10:39Z [Client ID: 02c9adfa-fab8-49fe-b448-98dee2ceeeed:5da4a8da-820f-4697-8c03-4ad1d3ca8b17]
2024-02-14T21:10:39Z [Cache Directory: /opt/<service-account-name>/.cache/sts]
2024-02-14T21:10:39Z [Client disabled: 02c9adfa-fab8-49fe-b448-98dee2ceeeed:5da4a8da-820f-4697-8c03-4ad1d3ca8b17]

I believe that sts should do one of the following depending on the developer's design philosophy:

1. Error out with a succinct and syslog-friendly single-line error message that explains that ~/.cache doesn't exist and needs to be created to allow sts to cache files, obfuscating the multi-line Go traceback.
2. Attempt to create the ~/.cache directory with the correct ownership and and appropriately conservative mode, probably 0700 to ensure we aren't creating a directory that STIG benchmarks would consider to be too open.

Thanks!