Better error handling when pg_stat_monitor has an unsupported version

Suggested implementation:

1. Check if versions is in right format (x.x.x) otherwise return error before split here: https://github.com/percona/pmm-agent/blob/master/agents/postgres/pgstatmonitor/pgstatmonitor.go#L125

User impact: User cannot see QAN data for Postgresql

STR:

1. Add PostgreSQL to PMM with specified pg_stat_monitor option
2. pg_stat_monitor should have unsupported version like 'devel'
3. Check pmm-agent.log in PMM server
   Actual result:

    panic: runtime error: index out of range [1] with length 1
   goroutine 190 [running]:
   github.com/percona/pmm-agent/agents/postgres/pgstatmonitor.getPGMonitorVersion(0xc0005b2180, 0xc000419380, 0xc0007a76b0, 0xc0007a72af)
   	/tmp/go/src/github.com/percona/pmm-agent/agents/postgres/pgstatmonitor/pgstatmonitor.go:126 +0x345
   github.com/percona/pmm-agent/agents/postgres/pgstatmonitor.(*statMonitorCache).getStatMonitorExtended(0xc0005b4450, 0x166f900, 0xc000418ae0, 0xc0005b2180, 0x1, 0xc000418b10, 0xc000418b40, 0x0, 0x0)
   	/tmp/go/src/github.com/percona/pmm-agent/agents/postgres/pgstatmonitor/stat_monitor_cache.go:86 +0x2da
   github.com/percona/pmm-agent/agents/postgres/pgstatmonitor.(*PGStatMonitorQAN).Run(0xc0007c2f60, 0x166f900, 0xc000418ae0)
   	/tmp/go/src/github.com/percona/pmm-agent/agents/postgres/pgstatmonitor/pgstatmonitor.go:140 +0xf4
   runtime/pprof.Do(0x166f900, 0xc000418ae0, 0xc0005b2300, 0x2, 0x2, 0xc000248b20)
   	/usr/local/go/src/runtime/pprof/runtime.go:40 +0xcc
   created by github.com/percona/pmm-agent/agents/supervisor.(*Supervisor).startBuiltin
   	/tmp/go/src/github.com/percona/pmm-agent/agents/supervisor/supervisor.go:456 +0x694

Expected result: More useful error message