Provide return string from PhaseSI() if phase can't be determined.

[henningjp]

Closes #1631

If phase can't be determined, because of an error in PropsSI() or the state point location, PhaseSI() returned a blank string:

In: PhaseSI("T",270.3895839,"P",264626.0542,"R134a")
Out: ''

Modified PhaseSI() to instead return "unknown", appended with any error messages from the call to PropsSI():

In: PhaseSI("T",270.3895839,"P",264626.0542,"R134a")
Out: 'unknown: Saturation pressure [264626 Pa] corresponding to T [270.39 K] is within 1e-4 % of given p [264626 Pa] : PropsSI("Phase","T",270.3895839,"P",264626.0542,"R134a")'

[henningjp]

@ibell, those error strings can be kind of long. What do you think?

[ibell]

I say keep the long error strings.

…

On Sat, Feb 10, 2018 at 4:40 PM, Jeff Henning ***@***.***> wrote: @ibell <https://github.com/ibell>, those error strings can be kind of long. What do you think? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#1645 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABxha5-k-yYXSIaalvJnu31iDRc7tITNks5tTij0gaJpZM4SA77i> .

[ibell]

Looks good to me!