MBS-7245: MBImport.pl: Return a non-0 exit status in case tar fails

The exit status was previously always zero, even if tars exit status was
>0. This made importing an initial dataset continue even if MBImport
failed.
INSTALL.md: Turn the suggested LANG2COUNTRY entries into a proper markdown list

Due to the missing empty line, the list was rendered as though it was a
continuation of the preceding sentence.
Merge remote-tracking branch 'mineo/mbs-7245' into beta-no-release-editor

* mineo/mbs-7245:
  MBS-7245: MBImport.pl: Return a non-0 exit status in case tar fails
Merge remote-tracking branch 'mineo/installmd-markdown-list' into beta-no-release-editor

* mineo/installmd-markdown-list:
  INSTALL.md: Turn the suggested LANG2COUNTRY entries into a proper markdown list
Merge branch 'beta-no-release-editor' into beta

* beta-no-release-editor:
  INSTALL.md: Turn the suggested LANG2COUNTRY entries into a proper markdown list
  MBS-7245: MBImport.pl: Return a non-0 exit status in case tar fails