Error printed when adding a model with a faulty credential can be improved

I attempted to add a model with an invalid credential:

$ juju add-model --credential mithrandir rivendell

Output:

Uploading credential 'google/pete/mithrandir' to controller
ERROR failed to create environ: Get https://www.googleapis.com/compute/v1/projects/juju-gce-1325?alt=json: private key should be a PEM or plain PKSC1 or PKCS8; parse error: asn1: structure error: tags don't match
 (2 vs {class:0 tag:16 length:13 isCompound:true}) {optional:false explicit:false application:false private:false defaultValue:<nil> tag:<nil> stringType:0 timeType:0 set:false omitEmpty:false} @5

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This error should be caught and the user presented with something more meaningful and tender.

Suggestion:

The model could not be created as credential 'mithrandir' is invalid.

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Note that if the "Uploading credential..." part gets printed we may want to assure the user that a bad credential has not ended up on the controller. I suggest suppressing that message until the credential has been successfully validated.