no useful error message when bootstrap fails due to ssh key issues

$ juju bootstrap aws
Creating Juju controller "aws-us-east-1" on aws/us-east-1
Looking for packaged Juju agent version 3.1-beta1 for amd64
No packaged binary found, preparing local Juju agent binary
Launching controller instance(s) on aws/us-east-1...
 - i-09f709d2cd34962a6 (arch=amd64 mem=4G cores=2)
Installing Juju agent on bootstrap instance
Waiting for address
Attempting to connect to x.x.x.x:22
Attempting to connect to x.x.x.x:22
ERROR failed to bootstrap model: cancelled
$

My bootstrap failed with no indication why.

It wasn't until trying again with debug I found the answer:

10:46:44 DEBUG juju.provider.common bootstrap.go:650 connection attempt for x.x.x.x failed: @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: UNPROTECTED PRIVATE KEY FILE! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
Permissions 0664 for '/home/ubuntu/.local/share/juju/ssh/juju_id_rsa' are too open.
It is required that your private key files are NOT accessible by others.
This private key will be ignored.
Load key "/home/ubuntu/.local/share/juju/ssh/juju_id_rsa": bad permissions
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: UNPROTECTED PRIVATE KEY FILE! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
Permissions 0644 for '/home/ubuntu/.ssh/id_rsa' are too open.
It is required that your private key files are NOT accessible by others.
This private key will be ignored.
Load key "/home/ubuntu/.ssh/id_rsa": bad permissions
ubuntu@x.x.x.x: Permission denied (publickey).

Juju should provide a reasonable error message when bootstrap fails.