Better error message if plugin jar can't be found

[Nicklas Nordborg]

The stacktrace that is shown when a trying to install a plugin and the JAR file can't be found is not very informative:

Could not instantiate plugin class
...
Caused by: java.util.zip.ZipException

We could probably let the JarClassLoader check that the JAR file exists and throw a better error message if it doesn't.

[Martin Svensson]

(In [3052]) Fixes #459 Better error message if plugin jar can't be found

[Nicklas Nordborg]

(In [3078]) References #459: Better error message if plugin jar can't be found

Throw InvalidDataException instead of IOException to make the web application display a simple error message instead of a complete stack trace.