fix: add flags, validations, error messages

[18alantom]

• fix: prevent no len of None
• fix: add sensible message if missing apps
• chore: bump bench version for frappe nightly and v15 (addresses Update bench version from 5.15.2 to latest #722)
• feat: press settings flags for app cache

[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (c865255) 45.25% compared to head (5992e0f) 45.29%.
Report is 4 commits behind head on master.

❗ Current head 5992e0f differs from pull request most recent head 7e4fea1. Consider uploading reports for the commit 7e4fea1 to get more accurate results

Files	Patch %	Lines
press/press/doctype/release_group/release_group.py	69.56%	7 Missing ⚠️
press/press/doctype/agent_job/agent_job.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1430      +/-   ##
==========================================
+ Coverage   45.25%   45.29%   +0.03%     
==========================================
  Files         312      312              
  Lines       20966    20987      +21     
==========================================
+ Hits         9489     9506      +17     
- Misses      11477    11481       +4     

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[18alantom]

Merging. Failing Server (1) is due to no space left in GActions Cache. Ignoring CodeCov.