Call sentry only when it was set up.

Fixes issue 1727.
Send traceback to log if Sentry not set up.
Correctly display errors in configure-pg-env.sh.

* Print errors in configure-pg-env.sh to STDERR instead of STDOUT.
* Don't run `export` in recreate-schema.sh until after
  configure-pg-env.sh is successfully ran, to avoid the case where
  it would run `export` without any arguments. `set -e` ensures it
  won't run the `export $ENV` line if configure-pg-env.sh fails.
Bump version to 10.1.51
Bump version to 10.1.51-dev
Disallow `/on/*` in robots.txt
Merge pull request #1755 from gittip/issue-1753

Disallow `/on/*` in robots.txt
Merge pull request #1730 from gittip/issue-1727

Call sentry only when it was set up.
Add decorator log_cursor - prints all sql to stdout.
Control if the homepage updater thread gets run.
Control the output of metrics to stdout.
Limit the number of transfers to 5 to speed up the test.
Add pytest-cache.

Adds the option to rerun only tests that failed in the last run.
Merge pull request #1756 from gittip/nicer-dev

Nicer dev environment
#1756: add gitignore for pytest-cache
Merge pull request #1748 from duckinator/build-system-fixes

Correctly display errors in configure-pg-env.sh.