basesrc: demote error to warning
basesrc: demote error to warning

This is not an error. A subclass returning FALSE for is_seekable() is one way of
saying that we can't operate in pull mode.

surround-io/gstreamer,krichter722/gstreamer,centricular/gstreamer,surround-io/gstreamer,Lachann/gstreamer,mparis/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,ensonic/gstreamer,surround-io/gstreamer,cfoch/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,ensonic/gstreamer,Lachann/gstreamer,shelsonjava/gstreamer,Lachann/gstreamer,centricular/gstreamer,cfoch/gstreamer,cablelabs/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,jpakkane/gstreamer,cfoch/gstreamer,jpxiong/gstreamer,cfoch/gstreamer,centricular/gstreamer,centricular/gstreamer,cablelabs/gstreamer,shelsonjava/gstreamer,ensonic/gstreamer,mparis/gstreamer,jpakkane/gstreamer,mparis/gstreamer,jpakkane/gstreamer,jpakkane/gstreamer,jpakkane/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,mparis/gstreamer,Lachann/gstreamer,cablelabs/gstreamer,cablelabs/gstreamer,Lachann/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,cablelabs/gstreamer,cfoch/gstreamer,mparis/gstreamer,surround-io/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,surround-io/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,ensonic/gstreamer,ensonic/gstreamer