[grid] make logging more precise

This is seen in an example application:

```
INFO:ecl2df.eclfiles:Opening data vectors from EGRID file: DROGON_HIST.EGRID
INFO:ecl2df.eclfiles:Opening grid data from EGRID file: DROGON_HIST.EGRID
INFO:ecl2df.grid:Extracting vectors ['FIPNUM', 'EQLNUM', 'SATNUM', 'SWATINIT', 'SWAT', 'PORO', 'PERMX', 'NTG', 'PCW', 'PPCW', 'PPCWMAXasada', 'SWL'] from INIT file
INFO:ecl2df.eclfiles:Opening INIT file: DROGON_HIST.INIT
INFO:ecl2df.grid:Extracting vectors ['FIPNUM', 'EQLNUM', 'SATNUM', 'SWATINIT', 'SWAT', 'PORO', 'PERMX', 'NTG', 'PCW', 'PPCW', 'PPCWMAXasada', 'SWL'] from RST file
```
* Not necessary to talk about EGRID file twice
* Say INIT file loading before which vectors extrcted.
* List only vectors that could be found
* Log a warning when a vector could not be found in INIT or RST.