SMSqueakMap behaves badly when attempting to install a nonexistent package.
This change set fixes the problem by adding some nil checks and by throwing
a Warning when attempting to load a package with a bogus name or UUID.

SUnit tests are provided in a companion change set.

Note: Most SM functions use #inform: to display warnings to the user.
I used #notify: in here because I am assuming that server instances
of SqueakMap may be running headless, and that a headless server could
handle Warning exceptions with no UI interaction. This seemed better
overall than just letting the SMSqueakMap>>installPackageNamed: and
SMSqueakMap>>installPackageWithId: methods fail silently.

Dave

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