initdb failures are reported as unreadable single-line exception dumps

[mbautin]

We need to print the actual stderr instead of an extremely long Python string with escaped end-of-line characters.

(end of standard error)
Traceback (most recent call last):
  File "/home/yugabyte/code/yugabyte-db/python/yb/gen_initial_sys_catalog_snapshot.py", line 78, in <module>
    main()
  File "/home/yugabyte/code/yugabyte-db/python/yb/gen_initial_sys_catalog_snapshot.py", line 68, in main
    shell=True
  File "/home/yugabyte/code/yugabyte-db/build/venv/lib/python3.7/site-packages/yugabyte_pycommon/external_calls.py", line 339, in run_program
    result.raise_error_if_failed()
  File "/home/yugabyte/code/yugabyte-db/build/venv/lib/python3.7/site-packages/yugabyte_pycommon/external_calls.py", line 146, in raise_error_if_failed
    raise ExternalProgramError(self.error_msg, self)
yugabyte_pycommon.external_calls.ExternalProgramError: ('Non-zero exit code 1 from external program {{ \'/home/yugabyte/code/yugabyte-db/build-support/run-test.sh\' /home/yugabyte/code/yugabyte-db/build/debug-gcc-dynamic-ninja/tests-pgwrapper/create_initial_sys_catalog_snapshot }} running in \'/home/yugabyte/code/yugabyte-db/build/debug-gcc-dynamic-ninja\', saving stdout to {{ /home/yugabyte/code/yugabyte-db/build/debug-gcc-dynamic-ninja/create_initial_sys_catalog_snapshot.out }}, stderr to {{ /home/yugabyte/code/yugabyte-db/build/debug-gcc-dynamic-ninja/create_initial_sys_catalog_snapshot.err }}.\nStandard output from external program {{ \'/home/yugabyte/code/yugabyte-db/build-support/run-test.sh\' /home/yugabyte/code/yugabyte-db/build/debug-gcc-dynamic-ninja/tests-pgwrapper/create_initial_sys_catalog_snapshot }} running in \'/home/yugabyte/code/yugabyte-db/build/debug-gcc-dynamic-ninja\', saving stdout to {{ /home/yugabyte/code/yugabyte-db/build/debug-gcc-dynamic-ninja/create_initial_sys_catalog_snapshot.out }}, stderr to {{ /home/yugabyte/co