* 'permission' instead of 'permissions'
* Also added a more user friendly error message when Attribute.string_to_permissions fails
```
TypeError: Attribute::permissions error:
Expected a string containing any of the keys, seperated by commas: READABLE,WRITEABLE,READ_REQUIRES_ENCRYPTION,WRITE_REQUIRES_ENCRYPTION,READ_REQUIRES_AUTHENTICATION,WRITE_REQUIRES_AUTHENTICATION,READ_REQUIRES_AUTHORIZATION,WRITE_REQUIRES_AUTHORIZATION
Got: 1
```
```
Exception: Error parsing Device Config's GATT Services. The key 'permission' must be renamed to 'permissions'
```
before:
```
File "/home/alanrosenthal/code/fitbit/bumble/bumble/gatt.py", line 572, in __str__
f'Descriptor(handle=0x{self.handle:04X}, '
File "/home/alanrosenthal/code/fitbit/bumble/bumble/att.py", line 756, in read_value
self.permissions & self.READ_REQUIRES_ENCRYPTION
TypeError: unsupported operand type(s) for &: 'str' and 'int'
```
