add a type check to xpath_search

[MichaelChirico]

Ran into this when making an interactive typo:

Error in xml_find_all.xml_node(., pg, "//div[contains(text(), 'Results')]") : 
  STRING_ELT() can only be applied to a 'character vector', not a 'list'

The STRING_ELT part is likely to be puzzling, so we catch this mistake one line earlier instead.

[jimhester]

Can you please add a bullet to NEWS? It should briefly describe the change and end with (@yourname, #issuenumber).

[MichaelChirico]

Done

[jimhester]