Display a friendlier error message when running outside the UK

[abhidg]

Currently CATS fails with unknown location if we supply a non-UK postcode (example shown is for Mountain View, CA, USA). This can happen if CATS is run outside the UK as ipapi.co returns the local postcode.

at 16:20 > cats --loc 94043 -d 30

The.____ ..... __ .... ________ . ______...
.. /  __)...../  \....(__    __).)  ____)....
..|  /......./    \......|  |...(  (___........
..| |limate./  ()  \ware.|  |ask.\___  \cheduler
..|  \__...|   __   |....|  |....____)  )....
...\    )..|  (..)  |....|  |...(      (..

WARNING:root:config file not found
WARNING:root:Unspecified carbon intensity forecast service, using carbonintensity.org.uk
Warning: truncating postcode 94043 to 94.
ERROR:root:Error: unknown location 94043

ERROR:root:Location should be be specified as the outward code,
for example 'SW7' for postcode 'SW7 EAZ'.