debug.log:
2017-02-22 21:15:33 Initializing databases...
2017-02-22 21:15:33 Unable to open file /home/mainnet/.bitcoin/blocks/blk00000.dat
2017-02-22 21:15:33 Unable to open file /home/mainnet/.bitcoin/blocks/blk00000.dat
2017-02-22 21:15:33 ERROR: WriteBlockToDisk: OpenBlockFile failed
2017-02-22 21:15:33 ERROR: LoadBlockIndex(): writing genesis block to disk failed

src/validation.cpp:1111
bool WriteBlockToDisk(const CBlock& block, CDiskBlockPos& pos, const CMessageHeader::MessageStartChars& messageStart)
{
    // Open history file to append
    CAutoFile fileout(OpenBlockFile(pos), SER_DISK, CLIENT_VERSION);
    if (fileout.IsNull())
        return error("WriteBlockToDisk: OpenBlockFile failed");

src/validation.cpp:3432
FILE* OpenDiskFile(const CDiskBlockPos &pos, const char *prefix, bool fReadOnly)
{
    if (pos.IsNull())
        return NULL;
    boost::filesystem::path path = GetBlockPosFilename(pos, prefix);
    boost::filesystem::create_directories(path.parent_path());
    FILE* file = fopen(path.string().c_str(), "rb+");
    if (!file && !fReadOnly)
        file = fopen(path.string().c_str(), "wb+");
    if (!file) {
        LogPrintf("Unable to open file %s\n", path.string());
        return NULL;
    }