Provide much better error messages

- When processes fail, they actually print a failure message
  on the user's terminal
- Regardless of success or failure, we print all output to
  /opt/tljh/installer.log

This should make debugging people's issues *much* easier, since
we can actually see the output of failing commands rather than
having to guess.

tljh/utils.py

@@ -0,0 +1,37 @@
+"""
+Miscelaneous functions useful in at least two places unrelated to each other
+"""
+import subprocess
+import logging
+
+
+# Copied into bootstrap/bootstrap.py. Make sure these two copies are exactly the same!
+def run_subprocess(cmd, *args, **kwargs):
+    """
+    Run given cmd with smart output behavior.
+
+    If command succeeds, print output to debug logging.
+    If it fails, print output to info logging.
+
+    In TLJH, this sends successful output to the installer log,
+    and failed output directly to the user's screen
+    """
+    logger = logging.getLogger('tljh')
+    proc = subprocess.run(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, *args, **kwargs)
+    printable_command = ' '.join(cmd)
+    if proc.returncode != 0:
+        # Our process failed! Show output to the user
+        logger.error(proc.stdout.decode())
+        e = Exception( 'command {command} failed with return code {code}'.format(
+            printable_command, proc.returncode
+        ))
+        logger.exception(e)
+        raise e
+    else:
+        # This goes into installer.log
+        logger.debug('Ran {command} with exit code {code}'.format(
+            command=printable_command, code=proc.returncode
+        ))
+        # This produces multi line log output, unfortunately. Not sure how to fix.
+        # For now, prioritizing human readability over machine readability.
+        logger.debug(proc.stdout.decode())