tests: Give more descriptive import error.
tests: Give more descriptive import error.

This code takes care of the environment running Python 3.4 when
test label is passed directly to the test-backend command:
./tools/test-backend test_alert_words

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