added check for file opening, error message if it doesn't
added check for file opening, error message if it doesn't

mne-tools/mne-cpp,chdinh/mne-cpp,LorenzE/mne-cpp,LorenzE/mne-cpp,mne-tools/mne-cpp,mne-tools/mne-cpp,chdinh/mne-cpp,mne-tools/mne-cpp,chdinh/mne-cpp,mne-tools/mne-cpp,LorenzE/mne-cpp,chdinh/mne-cpp,chdinh/mne-cpp,LorenzE/mne-cpp,chdinh/mne-cpp,mne-tools/mne-cpp