Previously vim would silently fail when help tags couldn't be generated.
We need to pass the "verbose" flag (with verbose level 1) to convince
vim to print errors to standard error.
I messed that up in my refactoring in #52767, since I moved the relevant
bash code out of a function and failed to adjust the shell variable
name. Now the plugin build will fail loudly when help tag generation
fails to make sure this doesn't happen again.
