Better error message for missing search field

[jefferis]

previously something like this:

neuprintr::neuprint_ids('rhubarb:.+')

would have given

Error in if (fieldtype == "STRING") { : argument is of length zero

now:

Error in neuprint_search(x, meta = F, field = "type", fixed = fixed, conn = conn,  : 
  Sorry, the `rhubarb` field doesn't exist in neuprint database (which is case-sensitive)!

[jefferis]

cc @AlexFragniere