Will fail if font does not have expected glyphs

[caternuson]

Not all fonts have all expected characters used to check height. Maybe add some try/except to catch this?

Adafruit CircuitPython 5.3.0 on 2020-04-29; Adafruit Circuit Playground Bluefruit with nRF52840
>>> from adafruit_bitmap_font import bitmap_font
>>> from adafruit_display_text import label
>>> font = bitmap_font.load_font("/monoMMM_5_90.bdf")
>>> my_label = label.Label(font, text="hello", color=0xFFFFFF)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "adafruit_display_text/label.py", line 122, in __init__
  File "adafruit_display_text/label.py", line 264, in _update_text
  File "adafruit_display_text/label.py", line 144, in _create_background_box
AttributeError: 'NoneType' object has no attribute 'height'
>>> glyph = font.get_glyph(ord("M"))
>>> type(glyph)
<class 'Glyph'>
>>> glyph = font.get_glyph(ord(" "))
>>> type(glyph)
<class 'NoneType'>
>>> glyph = font.get_glyph(ord("j"))
>>> type(glyph)
<class 'NoneType'>
>>> 

[caternuson]

Hopefully fixed with #51