A year ago I submitted a change to replace AnyError with tests for specific
errors (see https://github.com/elves/elvish/commit/87656c99). This does
something similar for DoesNotCompile. This ensures the test does what
is implied and makes correlating specific unit tests with compilation
errors easier.
This includes a couple of changes to compilation error messages to improve
readability (IMHO) but those are not the primary purpose of this change.
Related #1560
When attempting to update a read-only var return an error struct rather
than a simple string (i.e., a Go `error` type). This makes it possible
to include the var name in the error message. This builds on commit
a33ecb2d that highlights the offending var name in the stack trace but
does not include the var name in the error message. With this change the
error message includes the offending var name.
Related #255
