Add font file check

[caternuson]

oled_test.py (rst pin changed as needed)

import board
import digitalio
import adafruit_ssd1306

rst = digitalio.DigitalInOut(board.D22)

disp = adafruit_ssd1306.SSD1306_I2C(128, 64, board.I2C(), reset=rst, addr=0x3d)

disp.fill(0)
disp.fill_rect(5, 20, 30, 10, 1)
disp.text('hello world', 0, 0, 1)
disp.show()

CircuitPython

BAD Font File

Adafruit CircuitPython 4.1.0 on 2019-08-03; Adafruit Metro M4 Express with samd51j19
>>> import oled_test
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "oled_test.py", line 11, in <module>
  File "/lib/adafruit_framebuf.py", line 313, in text
  File "/lib/adafruit_framebuf.py", line 370, in __init__
RuntimeError: Invalid font file: font5x8.bin
>>> 

GOOD Font File

Adafruit CircuitPython 4.1.0 on 2019-08-03; Adafruit Metro M4 Express with samd51j19
>>> import oled_test
>>> 

Blinka

BAD Font File

pi@pizerow:~/oled_test $ python3 oled_test.py 
Traceback (most recent call last):
  File "oled_test.py", line 11, in <module>
    disp.text('hello world', 0, 0, 1)
  File "/home/pi/repos/Adafruit_CircuitPython_framebuf/adafruit_framebuf.py", line 313, in text
    self._font = BitmapFont()
  File "/home/pi/repos/Adafruit_CircuitPython_framebuf/adafruit_framebuf.py", line 370, in __init__
    raise RuntimeError("Invalid font file: " + font_name)
RuntimeError: Invalid font file: font5x8.bin
pi@pizerow:~/oled_test $

GOOD Font File

pi@pizerow:~/oled_test $ python3 oled_test.py 
pi@pizerow:~/oled_test $