Check whether the given configuration is valid.
If the configuration is valid, returns null.
Otherwise, returns a description of the reason
the configuration is invalid.
This should always return null if !inConfig(config).
Logs warnings associated with a valid config.
Returns false if the current platform must be skipped but
icons can still be generated for other platforms.
Returns true if icons can be generated for the current platform.
Logs warnings to out.
When called, inConfig(config) and isConfigValid(config) != null
are assumed to hold.