"API error: DB Error: constraint violation" when installing Stripe extension
I fixed this particular problem.  I haven't been able to recreate the problem so I can't state the source, but the following steps allowed me to install the extension.

1) remove all other extensions from the extensions folder (including ones which were not installed).
2) truncating the civicrm_extension  table
3) truncating the civicrm cache tables

Note that just trying v1.92 of the extension was not effective, nor was deleting the Stripe extension record in the civicrm_extension table.