gprecoverseg: log the error if pg_rewind fails

It didn't log the error message before if pg_rewind fails, fix that to make
DBA/field/developer's life eaisier.

Before this:
```
20201022:15:19:10:011118 gprecoverseg:earth:adam-[INFO]:-Running pg_rewind on required mirrors
20201022:15:19:12:011118 gprecoverseg:earth:adam-[WARNING]:-Incremental recovery failed for dbid 2. You must use gprecoverseg -F to recover the segment.
20201022:15:19:12:011118 gprecoverseg:earth:adam-[INFO]:-Starting mirrors
20201022:15:19:12:011118 gprecoverseg:earth:adam-[INFO]:-era is 0406b847bf226356_201022151031
```

After this:
```
20201022:15:33:31:019577 gprecoverseg:earth:adam-[INFO]:-Running pg_rewind on required mirrors
20201022:15:33:31:019577 gprecoverseg:earth:adam-[WARNING]:-pg_rewind: fatal: could not find common ancestor of the source and target cluster's timelines
20201022:15:33:31:019577 gprecoverseg:earth:adam-[WARNING]:-Incremental recovery failed for dbid 2. You must use gprecoverseg -F to recover the segment.
20201022:15:33:31:019577 gprecoverseg:earth:adam-[INFO]:-Starting mirrors
20201022:15:33:31:019577 gprecoverseg:earth:adam-[INFO]:-era is 0406b847bf226356_201022151031
```

gpMgmt/bin/gppylib/operations/buildMirrorSegments.py

@@ -394,6 +394,7 @@ class GpMirrorListToBuild:
             if not cmd.was_successful():
                 dbid = int(cmd.name.split(':')[1].strip())
                 self.__logger.debug("%s failed" % cmd.name)
+                self.__logger.warning(cmd.get_stdout())
                 self.__logger.warning("Incremental recovery failed for dbid %d. You must use gprecoverseg -F to recover the segment." % dbid)
                 rewindFailedSegments.append(rewindInfo[dbid].targetSegment)