[krbdev.mit.edu #5753] NM: BUG: APP: Do not report errors for modules that are not installed

[Jeffrey Altman via RT]

If an ImagePath value is not defined for a module, the module should be
ignored and an error loading the module should not be reported to the
user.  

There are many legitimate reasons why there might be a Module key for a
module that is not installed.  Examples include:

(1) Roaming profile and the module in question is installed on some
    machines but not others

(2) 64-bit and 32-bit environments.  A module might be installed in 
    one but not the other.

(3) The module was uninstalled and the user's profile was not available
    to perform cleanup.

In all of these cases, annoying the user with an error and incrementing
the error count on the module is wrong.  If the user wants to know why a
module is not present, she can look on the plugins page and easily see
that the module is not installed.