On Adobe XI Pro form tool how do I write a calculation such as excel formula =IFERROR(G14/C14,"")?

The script would be written by a programmer. Would you like resources to learn how to program JavaScript?

I think you're right. Could go further with

(A+0) == 0 ? 0 : (B+0) / (A+0) )

but it really isn't "simplified". Crucially, I think it would leave beginners more confused, not less...

No, I'm saying it IS valid. I tried it in Acrobat X... like all simplified field notation, it compiles directly into JavaScript, and this looks valid to me.

You can either check if the divisor is zero (or empty) first, or check if the result of the operation equals "Infinity" (no quotes). If that's the case, you know it was the result of division by zero, and then apply another value to it.

Keep in mind, though, that a percentage field can never be empty. Its default value is "0%", which is not always the correct result...

Acrobat allows a programmer to adds JavaScript code to do calculation and verification.  To handle the specific case here you would use getField to read the value of C14, and make sure it isn’t zero before dividing by it.