Make error message for non-capitalized contract names friendlier

[anton-trunov]

If the user tries to input foo where Blueprint expects a capitalized name they get

Error: Cannot create a contract with the name 'foo'

which often times leaves the users confused, see issue #105, for instance.

The error message should state the reason why it is rejected. An alternative would be to state the reason and offer to input the contract name once again in the interactive mode.