Need better error message when there are no hits

[joelnitta]

Successful query:

> gb_sequence_get(id = id_2, dnabin = TRUE)
2 DNA sequences in binary format stored in a list.

Mean sequence length: 104878 
   Shortest sequence: 16568 
    Longest sequence: 193188 

Labels:
KF683087
AC092140

Base composition:
    a     c     g     t 
0.293 0.240 0.208 0.260 
(Total: 209.76 kb)

Query with no hits returns unclear error message:

> gb_sequence_get(id = "x", dnabin = TRUE)
strsplit(sqs, "") でエラー: 文字列でない引数です