Add error messages if VST loading failed

[lukas-w]

From discussion in #4421 (comment). Currently when loading a VST fails, such as when trying to load a 64bit VST on a 32bit VST, no error message is shown. The user should be notified of the error and we should provide a reason for failing when possible. For the 32bit vs 64bit example, this means we should check if the user's computer and the LMMS installation support the plugin's architecture.

[Spekular]

Closing and consolidating into #5433