create a strict mode

[aslowwriter]

currently the error handling is a bit adhoc, and I want to solidify a bit more what happens in what circumstances. There are errors that we perhaps don't have to panic for (will expand list with things if I think of more):

1. unable to fetch external index
2. unknown reference
3. parsing a python file

However, because i've been burned a lot myself by overly permissive warnings, I want to have a strict mode where things that would otherwise only warn, will error. Some loose steps:

1. resolve error behaviour at startup
2. propagate those error settings
3. apply correct behaviour based on those settings when appropriate
4. document decisions

[aslowwriter]

see also #89