bugfix: lib/sh/db.sh: public_schema_exists(): don't hide the function call tree so it's clear which function is running the psql commands

fix: *{.sh,run}: stderr_matches() callers: added benign_error=1 where needed

fix: *{.sh,run}: stderr_matches() callers: usage: documented that they may require benign_error=1

fix: lib/sh/util.sh: stderr_matches(): usage: documented that this may require benign_error=1

fix: /README.TXT: Mac backup: exclude ~/VirtualBox VMs/Ubuntu/Ubuntu.vdi, to avoid it being re-uploaded twice each time, due to an rsync verification error (https://projects.nceas.ucsb.edu/nceas/issues/907)

bugfix: lib/sh/db.sh: pg_cmd(): updated for new echo_vars log_level

fix: lib/sh/db.sh: pg_schema_exists(): display the function name so it's clear which function is running the psql commands

fix: lib/sh/db.sh: pg_schema_exists(): don't use log++ because it hides the command that produces the benign error

/README.TXT: changed "then rerun with l=1 ..." to "then review diff, and rerun with `l=1` prepended" to ensure that user reviews diff before syncing

/README.TXT: to synchronize a Mac's settings with my testing machine's: removed separate step to upload just the VirtualBox VMs, because that is now part of the main upload