Calculate: Help text should not be copied as an answer for the next calculation

[greenfeld]

Seen on: OLPC 10.1.2 build 852/Sugar 0.84.16, as well as Fedora 12/Sugar 0.86.3

If a user does an action which implies to the calculator that it should use the previous result as the first value (such as the "-" sign for a negative number), help text should not be copied as a "result" for this purpose.

Reproduction:

1. Start the Calculate activity.
2. Type "help(y)" and press Enter. You should see that "No help about 'y' {is} available" for a "result".
3. Press any operand key (+, *, -, etc.)

Expected Action: The GUI should be smart enough to know that "No help about 'y' {is} available", so it should not try to run a calculation with that unless we are going to support string operations on such strings.

Actual Action: The help text is copied to the entry line and then the operand is appended, which makes no sense for calculation purposes.