Provide error message when storage path is invalid

I was getting:
[OServer]Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -1
  at java.lang.String.substring(String.java:1949)
  at com.orientechnologies.orient.server.OServer.loadStorages(OServer.java:518)
  at com.orientechnologies.orient.server.OServer.loadConfiguration(OServer.java:469)
  at com.orientechnologies.orient.server.OServer.startup(OServer.java:166)
  at com.orientechnologies.orient.server.OServer.startup(OServer.java:141)
  at com.orientechnologies.orient.server.OServer.startup(OServer.java:118)
  at com.orientechnologies.orient.server.OServerMain.main(OServerMain.java:31)

This is just a tweak to provide a better error message when someone doesn't fill in the storage path properly.

For example, path="/some/path" instead of path="local:/some/path".

I'm not too sure what the best way of logging the error is, so you might want to check/change that.
Merge pull request #1323 from markwoon/patch-1

Provide error message when storage path is invalid