You can not open the jp2-Files with the readProduct (as you can not open them in SNAP either, see Topic Sentinel 2 product opener). You need to open the following XML-File:
C:/
…’— S2
…’— S2A_OPER_PRD_MSIL1C_PDMC_…20151217T103953 <-- Your Directory of the S2-Dataset
…|---- AUX_DATA
…|---- DATASTRIP
…|---- GRANULE
…|---- HTML
…|---- rep_info
…|---- INSPIRE.xml
…|---- manifest.safe
…|---- S2A_OPER_BWI_MSIL1C_PDMC…20151217T103953.png
…’---- S2A_OPER_MTD_SAFL1C_PDMC…_20151217T103953.xml <-- open this XML-file in SNAP/snappy (readProduct)
Cheers,
Andreas