quickstart: Fix silent failure when .gitignore existsThis fixes issue… - #1583

Fixes #1582

I followed roughly the same steps in #1582 and wound up here. The problem I see with just changing the error message is the user then needs to

1. create/find their own asv.conf.json template file
   • the template isn't provided in the documentation, but all the fields are listed
   • it wasn't entirely clear where the config file went (top-level? benchmarks folder?), but that might have been just a misreading/misunderstanding on my part
2. Create a benchmarks folder and populate it with benchmarks.py and empty __init__.py.
3. Check/create their .gitignore to verify they're ignoring the same things (slightly out-of-date, standard python .gitignore with a typo).

Note that this bug will also occur if the benchmarks folder already exists, even if benchmarks.py/__init__.py don't.

Suggestions

1. The "template content already exists" message should be expanded to include what template content already exists.
2. Add link(s) to missing template files/contents somewhere
   • Since the contents of benchmarks.py immediately follow in the documentation, and they'll need to be overwritten anyway, this is less of an issue
3. Instead of modifying the .gitignore file, copy the missing files and leave the already existing files alone.

I think (1) is the minimum to fix the issue, but (2) and/or (3) would be very helpful.