crypt: try not to return "unexpected EOF" error

Before this change the code wasn't taking into account the error
io.ErrUnexpectedEOF that io.ReadFull can return properly. Sometimes
that error was being returned instead of a more specific and useful
error.

To fix this, io.ReadFull was replaced with the simpler
readers.ReadFill which is much easier to use correctly.

backend/crypt/cipher_test.go

@@ -1495,8 +1495,10 @@ func TestDecrypterRead(t *testing.T) {
 		case i == fileHeaderSize:
 			// This would normally produce an error *except* on the first block
 			expectedErr = nil
+		case i <= fileHeaderSize+blockHeaderSize:
+			expectedErr = ErrorEncryptedFileBadHeader
 		default:
-			expectedErr = io.ErrUnexpectedEOF
+			expectedErr = ErrorEncryptedBadBlock
 		}
 		if expectedErr != nil {
 			assert.EqualError(t, err, expectedErr.Error(), what)