inform user when NaN is returned from compute

[pkienzle]

No error is generated, and in fact the console still says "computation completed" when running the model with parameters that produce NaN.

When data is present an error is generated but only because the residual plot cannot be generated and the error is meaningless (actually wrong) as it suggests a plotting error due to log scale plot (even if the plot is linear).

2017-02-27 01:43:08 : Plotting Error: Data has no positive values, and therefore can not be log-scaled.
2017-02-27 01:43:08 : Plotting Error: Data has no positive values, and therefore can not be log-scaled.

The Compute button should check for NaN return values and indicate that on the console rather than reporting "computation completed".

Here is an example of the problem (more examples in the attached spreadsheet):

In hayter_msa if I set:
radius = 20Å
Temp = 298K
diel = 78
volfrac = 0.001
[salt] = 0.001 M

then vary the charge from 0 to infinity I get:

0-2 ⇒ all OK
3-32 ⇒ NaN
33-115 ⇒ all OK
116-195 ⇒ NaN
196-inf ⇒ all OK

[GitHub <noreply@…>]

In 8cec26bbf02ff352133f45ec024c74dc995b7a85/sasview:

Merge pull request #80 from SasView?/ticket-866

Ticket 866. As per discussion this morning this is approved and ready to merge so doing so now. closes #866