Add a new option ``hideProgress`` in ``waitMixin`` to hide
the progress, while waiting for changes to be ready.

The --run option in the enter command should ensure that the default
services have all started before executing the subcommand, as
specified in the help info.

This, however, does not seem to be the case as the current
implementation only sends a request to the API to start the default
services and immediately afterwards, executes the subcommand.
This can have undesired behavior. For example, the following fails:

    $ pebble enter --run stop srv1
    2023-06-01T09:52:52.516Z [pebble] Started daemon.
    2023-06-01T09:52:52.546Z [pebble] POST /v1/services 29.059682ms 202
    2023-06-01T09:52:52.546Z [pebble] Started default services with change 386.
    2023-06-01T09:52:52.565Z [pebble] Service "srv1" starting: <cmd..>
    2023-06-01T09:52:52.584Z [pebble] POST /v1/services 37.51026ms 202
    error: cannot perform the following tasks:
    - Stop service "srv1" (cannot stop service while starting)

It may also happen that the stop subcommand is executed, before the
service hasn't even been initiated/started.

    $ pebble enter --run stop srv99
    2023-06-22T05:46:02.604Z [pebble] Started daemon.
    2023-06-22T05:46:02.619Z [pebble] Service "srv55" starting: <cmd..>
    2023-06-22T05:46:02.629Z [pebble] POST /v1/services 23.863368ms 202
    2023-06-22T05:46:02.630Z [pebble] Started default services with change 9.
    2023-06-22T05:46:02.637Z [pebble] Service "srv1" starting: <cmd..>
    2023-06-22T05:46:02.646Z [pebble] POST /v1/services 14.761043ms 202
    2023-06-22T11:46:02+06:00 INFO Service "srv99" has never been started.

This commit should ensure that the default services have all started,
before executing the subcommand. It does so by ensuring the "change"
made by the "autostart" request is ready, before running the
subcommand. The status of the "change" can be "Error" once it's ready;
the subcommand will still be executed after.

According to the feedback from @virtustom, log the error messages from
``waitMixin.wait()`` if there are any.

Exit with non-zero error code if waiting for the "change" (of default
services start) to be ready returns any errors.

This commit adds a new test which checks that default services
start before executing subcommand.

Go 1.14 does not support os.Readfile(), use ioutil.Readfile() instead.

Instead of writing errors directly to os.Stderr in rcmd.Run,
write them to cli.Stderr which eventually points to os.Stderr;
but useful for testing purposes.

Test if default service start errors are properly reported and
if pebble exits with non-zero error codes. This is only tested
for the following command, for now:

    pebble enter --run [enter-OPTIONS..] <subcommand> [args..]

This reverts commit 1281c96
and reverts the hard-error behaviour introduced in
4098a71. Pebble should no longer,
at least not due to this commit, exit with non-zero error code if
waiting for the "change" (of default services start) to be ready
returns any errors.