NullPointerException when database data directory hasn't the proper permissions

[carlos-rodrigues-8x8]

What

When the database directory doesn't have the read/write permissions the server fails the startup with following error:

java.lang.NullPointerException: Cannot read the array length because "<local5>" is null

	at com.arcadedb.engine.FileManager.<init>(FileManager.java:83)
	at com.arcadedb.engine.FileManagerTest.construtor_failure_noPermissionsDirectory(FileManagerTest.java:34)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)

The error doesn't give enough context to user.

How

• Check if the directory has the proper permissions;
• If not throws IllegalArgumentException with the context;

[lvca]

Thanks @carlos-rodrigues-8x8 for the PR!