More descriptive declaration errors

[mmyros]

Currently, having any capital letters in table definition raises:

DataJointError: Declaration error in position 5 in line:
  sigmaMask:                                                     int#
Expected ":"

It would be more intuitive to raise a more specific error that would alert user about capital letters in table definition.

Steps to reproduce:

@schema
class Example(dj.Computed):
    definition = '''
    CamelBack: int
'''
Example()