Sliderule custom scale trap errors

[tonyforster]

Trap errors and give some feedback, eg onscreen display of error text
untrapped errors:

Syntax error: unexpected EOF (can't replicate)

Type error: unsupported operand types

• none and int (log where min =0) + int and string (int(x)+".0") hex() argument cant be converted (real x)

[walter]

See  http://bugs.sugarlabs.org/attachment/ticket/2326/Sliderule-17.xo

I do the error handling differently now so I can catch them and display them.

[tonyforster]

Still getting Syntax error: unexpected EOF
eg position function = log(x,10)/

[walter]

Ooops. Forgot to look for Syntax Errors... easy enough to add.

[walter]

 http://bugs.sugarlabs.org/attachment/ticket/2326/Sliderule-17.xo has a check for syntax errors

[tonyforster]

Attribute error:
when entering a formula into step 1/1.6

[walter]

This is actually two bugs: (1) a bug I introduced in creating the second custom slide; and (2) a problem on that I currently don't allow anything but numbers in the min, max, and step fields. The attachment with #2326 fixes #1. #2 is really a new feature request, not relevant to this ticket.