Add checks to make sure local files or DB exists at a user-specified path

[andybega]

Add a check so this becomes more informative:

> set_icews_opts("foo", TRUE, TRUE)
> read_icews()
Error in `$<-.data.frame`(`*tmp*`, "year", value = NA_integer_) : 
  replacement has 1 row, data has 0
In addition: Warning messages:
1: Unknown or uninitialised column: 'event_date'. 
2: In read_icews_raw(find_raw(), n_max) :