Support custom error message in assertion module

[LINKIWI]

This PR proposes the addition of an optional secondary argument to ctx.assert(...) which enables unit test callsites to specify a custom assertion failure error message.

This looks like, for example:

def test_foo(ctx):
    ctx.assert(False)

def test_foo_message(ctx):
    ctx.assert(False, "foo error message")

Executing these tests results in:

[foo.sky:2:15] assertion failed
...

[foo.sky:4:15] assertion failed: foo error message
...

I added corresponding unit test coverage.

$ bazelisk test --cache_test_results=no //go/...
INFO: Invocation ID: bc5157d2-13f6-4d45-84b3-42aae65fc926
INFO: Analyzed 10 targets (0 packages loaded, 137 targets configured).
INFO: Found 5 targets and 5 test targets...
INFO: Elapsed time: 2.453s, Critical Path: 0.51s
INFO: 6 processes: 1 internal, 5 processwrapper-sandbox.
INFO: Build completed successfully, 6 total actions
//go/assertmodule:assertmodule_test                                      PASSED in 0.3s
//go/hashmodule:hashmodule_test                                          PASSED in 0.4s
//go/protomodule:protomodule_test                                        PASSED in 0.5s
//go/urlmodule:urlmodule_test                                            PASSED in 0.4s
//go/yamlmodule:yamlmodule_test                                          PASSED in 0.3s