improve NameChecker c.3217insT error

[jtdendunnen]

Using NM_000492.3:c.3217insT in the NameChecker gives:
The "^" indicates the position where the error occurred.

Suggest to make the error message more informative, e.g.;
The "^" indicates the position where the error occurred. An insertion must be BETWEEN two flanking nucleotides, e.g. NM_004006.2:c.123_124insG.