make failsafe if you feed checkPDF or checkHTML another article type

[MicheleNuijten]

example:

checkPDF() # chose a html file
PDF error: May not be a PDF file (continuing anyway)
PDF error (2): Illegal character <21> in hex string
PDF error (4): Illegal character <4f> in hex string
PDF error (6): Illegal character <54> in hex string
PDF error (7): Illegal character <59> in hex string
PDF error (8): Illegal character <50> in hex string
PDF error (11): Illegal character <68> in hex string
PDF error (12): Illegal character <74> in hex string

(etc.)