Distinguishing between unset env. var. and missing file in system test.

[dhermes]

Fixes #2084.

/cc @geigerj

---

Sample output

$ env | grep GCLOUD $ env | grep GOOG $ $ python system_tests/run_system_test.py --package datastore D0811 08:35:27.609354178 9997 ev_posix.c:101] Using polling engine: poll To run the system tests, you need to set some environment variables. Please check the CONTRIBUTING guide for instructions. Missing variables: GCLOUD_TESTS_PROJECT_ID, GOOGLE_APPLICATION_CREDENTIALS $ $ GOOGLE_APPLICATION_CREDENTIALS=bye GCLOUD_TESTS_PROJECT_ID=hi \ > python system_tests/run_system_test.py --package datastore D0811 08:35:13.398294691 9979 ev_posix.c:101] Using polling engine: poll To run the system tests, you need to set some environment variables. Please check the CONTRIBUTING guide for instructions. The GOOGLE_APPLICATION_CREDENTIALS path 'bye' is not a file. $ $ GOOGLE_APPLICATION_CREDENTIALS=bye \ > python system_tests/run_system_test.py --package datastore D0811 08:35:19.764175549 9988 ev_posix.c:101] Using polling engine: poll To run the system tests, you need to set some environment variables. Please check the CONTRIBUTING guide for instructions. Missing variables: GCLOUD_TESTS_PROJECT_ID The GOOGLE_APPLICATION_CREDENTIALS path 'bye' is not a file. $ $ GOOGLE_APPLICATION_CREDENTIALS=setup.py \ > python system_tests/run_system_test.py --package datastore D0811 08:35:37.601515289 10006 ev_posix.c:101] Using polling engine: poll To run the system tests, you need to set some environment variables. Please check the CONTRIBUTING guide for instructions. Missing variables: GCLOUD_TESTS_PROJECT_ID

[tseaver]

LGTM