While adding the support in "promise: Support ssl_certificate in
check_surykatka_json" coverage for error reporting was missing, which has been
extended in this fix.

/reviewed-on !78

This is another fixup for commit
1c8269b2.

This will lead to minimise amount of entries, and they will be only shown
when the status would change.

Improvements:

 * total_seconds from surykatka is parsed
 * failure-amount is configurable
 * various smaller bugfixes

/reviewed-on nexedi/slapos.toolbox!77

Result of the promise is not important for instantiation.

Sites with bad certificates are reported with null values in surykatka
output, thus they have to be handled correctly.

It means that the server didn't reply at all.

Show URL only once and having this in mind simplify messages still keeping
all information.

It is only checked when maximum-elapsed-time is passed and surykatka is
recent enough to report total_seconds.

Also improve reporting even more to provide readable information for those
massive promises with ERROR/OK mixed states.

Prefix with ERROR and OK messages for problems and ok checks, as they are
grouped together in the output.

This is another fixup for commit
1c8269b2.

This is not covered by tests since they use a fake_send method.

Additionally improve python3 support as it is required for proper
cleanup procedure.

/reviewed-on nexedi/slapos.toolbox!74

The file_list will be iterated more than once, and filter in python3
is being consumed, so create a list instead for many iterations.

os.errno does not exists since python 3.7

Fixed check_surykatka_json for http-only sites

ssl_certificate is not important for http only checks.

Improvements:

 * support for ssl_certificate entry from surykatka
 * explanation of extended status codes
 * assurance that URLs are clickable in the monitor.app
 * format times for humans

/reviewed-on nexedi/slapos.toolbox!73

"Extended" status codes, like 524, 526 and others, come from surykatka
itself and shall be amended with additional explanation

Since standard date format is used in the ssl_certificate checks, do the
same for readability and consistency in bot_status.

/reviewed-on nexedi/slapos.toolbox!72

in https://lab.nexedi.com/nexedi/slapos/blob/master/stack/monitor/templates/monitor-httpd.conf.in there is no such thing as `/share/public/` URL

/reviewed-on !71