[MacPorts] #53174: buildbot: better handle removal of ports in mpbb

[MacPorts]

#53174: buildbot: better handle removal of ports in mpbb
----------------------------+---------------------
 Reporter:  mojca           |      Owner:  admin@…
     Type:  defect          |     Status:  new
 Priority:  Low             |  Milestone:
Component:  server/hosting  |    Version:
 Keywords:  buildbot        |       Port:
----------------------------+---------------------
 Here's an [https://build.macports.org/builders/ports-10.7_x86_64_legacy-
 watcher/builds/3863 example] of removal of a port that resulted in an
 unexpected error:
 {{{
 ./mpbb/mpbb: error: None of the specified ports were found in the port
 index.
 ./mpbb/mpbb: error: `list-subports' failed to run successfully
 }}}
 Our handling of removal of ports should probably be improved.

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Ticket URL: <https://trac.macports.org/ticket/53174>
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