Add a more verbose error message for "unable to open Buffered file "

D 35 CAL0002: Failed to open file: /var/lib/columnstore/data1/systemFiles/dbrm/SMTxnID, exception: unable to open Buffered file 

BufferedFile::BufferedFile(const char* fname, const char* mode, unsigned opts) :

    IDBDataFile( fname ),

    m_fp( 0 ),

    m_buffer( 0 )

{

    m_fp = fopen(fname, mode);

    if ( m_fp == NULL )

    {

        throw std::runtime_error("unable to open Buffered file ");

    }

Upon successful completion, fopen() shall return a pointer to the object controlling the stream. Otherwise, a null pointer shall be returned, [CX] [Option Start]  and errno shall be set to indicate the error.