Improve error messages when instantiating an SRAFileReader

[lbergelson]

Currently constructing a new SRAFileReader calls SRAAccession.isValid() and throws an exception saying Invalid SRA accession was passed to SRA reader if isValid returns false.

Since is valid can fail for a number of reasons, this is not a very helpful message. It's particularly not helpful because it could actually be a valid accession, but SRA is just not supported.

This should be improved so that someone instantiating an SRAReader can tell WHY it was considered invalid.

[lbergelson]

@a-nikitiuk @kwrodarmer Can you take this one please.

[kwrodarmer]

Yes, we have already been working on a new reporting framework for this, as you have suggested, based upon various exception types.

[cmnbroad]

Can this be closed (fixed in #638 ?)

[a-nikitiuk]

Yes, it should be

[cmnbroad]

Closed via #638.