open my $fh ... or continue with broken filehandle

While on Helpdesk debugging a problem probably caused by incorrectly calling this code, I noticed
  https://github.com/cancerit/vafCorrect/blob/5.7.2/perl/lib/Sanger/CGP/Vaf/Data/ReadVcf.pm#L102

```
    open my $fai_fh , '<', $self->{'_g'}.'.fai';
    while (<$fai_fh>) {
```

If the `.fai` file is not found, then this error will not be reported in a meaningful way.