Error handling tweaks

[acdha]

I ran into a couple of things while testing this:

• Using the profile name from the environment avoids needing to manage it in code
• I consolidated most of the error handling code into a single exception block which avoids some of the edge cases where e.g. it would attempt to delete an IGW even if detaching it failed for some reason
• Regions which cannot be accessed using the active credentials display an error but don't cause the program to exit