css linter: Fix error reporting.
css linter: Fix error reporting.

Three changes:
    * Change wording of error message.
    * Flush standard error.
    * Use unified diff.

rht/zulip,hackerkid/zulip,eeshangarg/zulip,timabbott/zulip,timabbott/zulip,jackrzhang/zulip,rht/zulip,shubhamdhama/zulip,zulip/zulip,hackerkid/zulip,jackrzhang/zulip,synicalsyntax/zulip,shubhamdhama/zulip,jackrzhang/zulip,eeshangarg/zulip,rht/zulip,timabbott/zulip,rht/zulip,hackerkid/zulip,timabbott/zulip,synicalsyntax/zulip,showell/zulip,showell/zulip,zulip/zulip,andersk/zulip,showell/zulip,shubhamdhama/zulip,brainwane/zulip,rishig/zulip,shubhamdhama/zulip,synicalsyntax/zulip,eeshangarg/zulip,synicalsyntax/zulip,showell/zulip,zulip/zulip,kou/zulip,andersk/zulip,kou/zulip,punchagan/zulip,tommyip/zulip,rishig/zulip,rht/zulip,kou/zulip,hackerkid/zulip,rht/zulip,punchagan/zulip,brainwane/zulip,synicalsyntax/zulip,dhcrzf/zulip,showell/zulip,kou/zulip,andersk/zulip,kou/zulip,brainwane/zulip,rishig/zulip,tommyip/zulip,andersk/zulip,zulip/zulip,tommyip/zulip,eeshangarg/zulip,jackrzhang/zulip,shubhamdhama/zulip,jackrzhang/zulip,shubhamdhama/zulip,tommyip/zulip,andersk/zulip,tommyip/zulip,rht/zulip,brainwane/zulip,dhcrzf/zulip,punchagan/zulip,tommyip/zulip,rishig/zulip,rishig/zulip,timabbott/zulip,hackerkid/zulip,zulip/zulip,jackrzhang/zulip,synicalsyntax/zulip,dhcrzf/zulip,punchagan/zulip,punchagan/zulip,kou/zulip,rishig/zulip,jackrzhang/zulip,timabbott/zulip,punchagan/zulip,showell/zulip,showell/zulip,tommyip/zulip,zulip/zulip,hackerkid/zulip,brainwane/zulip,timabbott/zulip,eeshangarg/zulip,dhcrzf/zulip,shubhamdhama/zulip,brainwane/zulip,synicalsyntax/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,brainwane/zulip,rishig/zulip,dhcrzf/zulip,dhcrzf/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,dhcrzf/zulip,zulip/zulip