fix(nimbus): fix retention displaying incorrect values

[moibra05]

Because

• We were not showing week 2 retention on the main page for experiments that ran for less than 2 weeks and for those still in the observation phase

This commit

• Adds an analysis error if an experiment was running for less than 2 weeks and was thus unable to get week 2 retention data
• Applies a similar approach to fix(nimbus): fix 3 days retention showing incorrect values #15287 where we only keep the second window index in the weekly analysis basis for retention so that we are not displaying incorrect values

Fixes #12647

[moibra05]

Once merged, this unblocks #15239