[lit] Fix handling of various keyword parse errors

In TestRunner.py, D78589 extracts a `_parseKeywords` function from
`parseIntegratedTestScript`, which then expects `_parseKeywords` to
always return a list of keyword/value pairs.  However, the extracted
code sometimes returns an unresolved `lit.Test.Result` on a keyword
parsing error, which then produces a stack dump instead of the
expected diagnostic.

This patch fixes that, makes the style of those diagnostics more
consistent, and extends the lit test suite to cover them.

Reviewed By: ldionne

Differential Revision: https://reviews.llvm.org/D81665