provision: Simplify messaging in tools/provision.

The script now outputs bullet points to the user when
it fails, and there are some basic comments at the top
of the file.  I also fixed the path of the log file.

Fixes #3230

tools/provision

@@ -1,4 +1,9 @@
 #!/bin/bash
+
+# Use this script to provision dependencies for your Zulip installation.
+# This script is idempotent, so it can be restarted at any time, and it
+# will usually run fairly quickly when your dependencies are up to date.
+
 set -e
 if [ "$EUID" -eq 0 ]; then
     echo "Error: The provision script must not be run as root" >&2
@@ -25,9 +30,10 @@ if [ $failed = 1 ]; then
     echo -e "\033[0;31m"
     echo "Provisioning failed!"
     echo
-    echo -n "The provision process is designed to be idempotent, so you can retry "
-    echo -n "after resolving whatever issue caused the failure (there should be a traceback above). "
-    echo -n "A log of this installation is available in zulip/log/provision.log"
+    echo "* Look at the traceback(s) above to find more about the errors."
+    echo "* Resolve the errors or get help on chat."
+    echo "* If you can fix this yourself, you can re-run tools/provision at any time."
+    echo "* Logs are here: zulip/var/log/provision.log"
     echo -e "\033[0m"
     exit 1
 fi