Also added error checking when parsing the database file.

Even recent versions of stdlibc++ doesn't mark the move constructor of
std::string as noexcept. Thus CompileCommands move operations cannot be
noexcept and copy operations must be supported for CompileCommands to be
used with standard contrainers.

Added a guessing algorithm based on the basename of the file. If there
is no file with the same basename, a file with similar directory and
filename is sought after.