fixed issue loading kdb files with key file

improved error message by adding messages to loading exceptions in Java

src/java/KP2AKdbLibrary/src/com/keepassdroid/database/exception/InvalidDBSignatureException.java

@@ -26,7 +26,7 @@ public class InvalidDBSignatureException extends InvalidDBException {
 	private static final long serialVersionUID = -5358923878743513758L;
 
 	public InvalidDBSignatureException() {
-		super();
+		super("Invalid database signature");
 	}
 
 }

src/java/KP2AKdbLibrary/src/com/keepassdroid/database/exception/InvalidKeyFileException.java

@@ -25,6 +25,10 @@ public class InvalidKeyFileException extends InvalidDBException {
 	private static final long serialVersionUID = 5540694419562294464L;
 
 	public InvalidKeyFileException() {
-		super();
+		super("invalid key file!");
+	}
+	
+	public InvalidKeyFileException(String msg) {
+		super(msg);
 	}
 }

src/java/KP2AKdbLibrary/src/com/keepassdroid/database/exception/InvalidPasswordException.java

@@ -31,6 +31,6 @@ public class InvalidPasswordException extends InvalidDBException {
 	}
 	
 	public InvalidPasswordException() {
-		super();
+		super("Invalid key!");
 	}
 }

src/java/KP2AKdbLibrary/src/com/keepassdroid/database/exception/KeyFileEmptyException.java

@@ -25,6 +25,6 @@ public class KeyFileEmptyException extends InvalidKeyFileException {
 	private static final long serialVersionUID = -1630780661204212325L;
 
 	public KeyFileEmptyException() {
-		super();
+		super("key file is empty!");
 	}
 }