Errors seen whilst compiling with AdaFruit Motor Shield ("invalid library")
Arduino IDE 1.6.6 and higher are picky about what you have in your libraries folders. There are two general valid library formats, the old "pre-1.5" format and the newer 1.5 format, the specification of which is at https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5:-Library-specification.

Any folder in any of the Arduino libraries folder that doesn't follow a valid library format will cause the "Invalid library" message. Common causes of this issue are:


Putting a sketch folder in the libraries folder. Libraries may
contain example sketches but a sketch by itself is not allowed.
Libraries that are in a subfolder, rather than the library folder
being in the root of the libraries folder as required.


Now lets look at your specific messages:

Invalid library found in C:\arduino-1.6.8\libraries\Adafruit-Motor-Shield-for-Arduino-master: C:\arduino-1.6.8\libraries\Adafruit-Motor-Shield-for-Arduino-master


By looking at the Adafruit-Motor-Shield-for-Arduino repository we see that it's not a library but the PCB design files for the shield and thus has no business being in the libraries folder. Please move it to a different location.

Invalid library found in C:\arduino-1.6.8\libraries\robotarm: C:\arduino-1.6.8\libraries\robotarm


Without being able to see what's in this folder I can't say exactly the problem. If you will provide a directory listing I could update this answer with specific directions. As it is now, that folder is doing you no good in the libraries folder.