AbstractChecker: ensure location exists

The default behavior for file format compatibility requires that the
provided location exists to read from, using the Location.exists()
method.
OBFFormat: verify stream is valid before reading

The OBFFormat just starts reading from an IRandomAccessStream without
checking if it actually contains enough bytes to read from.

So, we should use the validStream method first to ensure its capacity.
This also will avoid reading from VirtualHandles.
DICOMFormat: ensure source exists

The DICOMFormat Checker overrides isFormat(String, SCIFIOConfig) so it
needs to explicitly check that the id exists when determining
compatibility.
JPEGFormat: ensure source exists

The JPEGFormat Checker overrides isFormat(String, SCIFIOConfig) so it
needs to explicitly check that the id exists when determining
compatibility.