Add constraints
no more hardcoded raster column name
Remove check against command output for string 'error'.

There could be a false positive like in:
```
NOTICE:  Returned error message: numeric field overflow (22003)
```
For single-band rasters, create indexes on min/max values.
Doing so would greatly speedup the queries to fetch min/max pixel
values for use by a raster colorizer wizard.