rustbuild: Improve error messaging in bootstrap.py
rustbuild: Improve error messaging in bootstrap.py

For normal invocations, print a short error message and exit. When
the verbose option is enabled, also print the backtrace.

aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust