Fixes scanning on duplex scanners

[fewebahr]

Some scanners (e.g. the Fujitsu ScanSnap S1500) scan both sides of a document in a single pass.

The result is that the one operation on a duplex scanner can result in 2 images, if the scanned document is in fact 2-sided. Each image may have on or multiple frames depending on the Format, just like in simplex.

Right now, the ReadImage function calls *Conn.Cancel() every time *Conn.ReadImage is invoked. This is safe for a simplex scanner-- but for a duplex scanner, it cancels the operation, losing the second image created by the second side of the document. The result is that each call to *Conn.ReadImage returns only the front side of each document.

This diff fixes the problem. It is only necessary to call *Conn.Cancel if an error was encountered (in which case it is acceptable that the second side is lost). This way, *Conn.ReadImage can be invoked multiple times; once for the front, and once for the back (if any).

[fewebahr]

See #18

[nint8835]

This PR also fixes scanning from scanners equipped with an automatic document feeder (my current use-case), any chance this PR could be merged @tjgq?