Config.add_font_file() does not check for font file

[ashutoshvarma]

When adding font in pyxpdf with new Config.add_font_file() method if font_path is incorrect(file does not exist) no error is thrown.

Steps to Reproduce:-

from pyxpdf import xpdf as x
doc = x.Document('samples/simple1.pdf')
iout = x.RawImageOutput(d)
iout.get(0).show()                # Output: Syntax Error: Couldn't find a font for 'Helvetica'

x.Config.add_font_file('Helvetica', 'asbajd')
iout.get(0).show()                # Output: Syntax Error: Couldn't find a font for 'Helvetica', Still same error

xpdf error logs are not very clear, we should check whether file exists and is readable in Config.add_font_file() and throw error with appropriate message

[ashutoshvarma]

added commit which checks for file, but it does not check whether its a valid font file.