Fix Bug 815: GL*: Change glIs<Buffer>Enabled() -> glIs<Buffer>Bound() to reflect semanics - Also fix the exception message (enabled/disabled -> bound/unbound)

Reason of change: Avoid confusion and point to the cause!

API change:
  glIsVBOArrayEnabled() -> glIsVBOArrayBound()
  glIsVBOElementArrayEnabled() -> glIsVBOElementArrayBound()
  glIsPBOPackEnabled() -> glIsPBOPackBound()
  glIsPBOUnpackEnabled() -> glIsPBOUnpackBound()

Exception message change:
  "must be enabled to call this method" -> "must be bound to call this method"
  "must be disabled to call this method" -> "must be unbound to call this method"