When commands are batched, the failure message can't include the command
group label with buildbox-run.

This adds a timed activity message to bring batch command logging for
REAPI-based sandboxes in line with the default sandbox as far as
possible.

Thix fixes 3 xfails with buildbox-run.

This will be used to improve batch command logging with REAPI.

scheduler.py: Handle exceptions that are caught under the event loop

Closes #1245

See merge request BuildStream/buildstream!1788

The default exception handler of the async event loop bypasses
our custom global exception handler. Ensure that isn't the case,
as these exceptions are BUGS & should cause bst to exit.

_frontend/widget.py: show_pipeline() Don't show buildable for junction

Closes #1205

See merge request BuildStream/buildstream!1764

Instead output "junction" in magenta for info. Also include test
in format/junctions.py

Make PipelineSelection a proper enum type

Closes #1135

See merge request BuildStream/buildstream!1787

PipelineSelection is one of the few stringy types that weren't
converted to FastEnum, presumably because we lacked a mechanism for
only allowing a sub-set of options as CLI arguments.

We've re-designed this since, and as part of the UI/UX refactor we'd
like to generally clean this, but that is probably still a while out.

Since that hasn't happened, for now, this adds a feature to the
FastEnumType that allows specifying only a subset of values is allowed
for a specific command, so that we can use the type as a proper
enum.

We also get rid of a number of accidental uses of strings, and move
PipelineSelection to buildstream.types so that we don't have a
significant import overhead for it.