If the user mistypes a build type or architecture name, that leads to a raw unhandled exception:
```
$ sudo ./build-vyos-image --build-type no-such-type generic
I: Checking if packages required for VyOS image build are installed
I: using build flavors directory data/build-flavors
Traceback (most recent call last):
File "/vyos/./build-vyos-image", line 295, in <module>
with open(make_toml_path(defaults.BUILD_TYPES_DIR, pre_build_config["build_type"]), 'rb') as f:
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: 'data/build-types/no-such-type.toml'
```
A more user-friendly error could be nice.