Suppress error in sp-point-in-string/comment

`syntax-ppss` may emit an error.  So ignore it.
Do not print error message when in minibuffer
Merge pull request #43 from tkf/point-in-string-ignore-error

Suppress error in sp-point-in-string/comment
Merge pull request #44 from tkf/sp-get-sexp-minibufferp

Do not print error message when in minibuffer