provision: Clean up error message for missing virtualenv.
provision: Clean up error message for missing virtualenv.

This new message should be clearer; it does come up occasionally.

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