'az image create' should fail if image name already exists

When running `az image create`, if an image with the same name already exists it should fail.
What happens today is that the image is not created, but the command finishes successfully and shows the info of the old image.
---

### Environment summary
```
azure-cli (2.0.23)

acr (2.0.17)
acs (2.0.22)
advisor (0.1.0)
appservice (0.1.22)
backup (1.0.3)
batch (3.1.7)
batchai (0.1.3)
billing (0.1.6)
cdn (0.0.10)
cloud (2.0.10)
cognitiveservices (0.1.9)
command-modules-nspkg (2.0.1)
configure (2.0.12)
consumption (0.2.0)
container (0.1.15)
core (2.0.23)
cosmosdb (0.1.15)
dla (0.0.15)
dls (0.0.18)
eventgrid (0.1.5)
extension (0.0.6)
feedback (2.0.6)
find (0.2.7)
interactive (0.3.11)
iot (0.1.15)
keyvault (2.0.15)
lab (0.0.13)
monitor (0.0.13)
network (2.0.19)
nspkg (3.0.1)
profile (2.0.16)
rdbms (0.0.9)
redis (0.2.10)
reservations (0.1.0)
resource (2.0.19)
role (2.0.15)
servicefabric (0.0.7)
sql (2.0.17)
storage (2.0.21)
vm (2.0.20)

Python location '/usr/local/opt/python3/bin/python3.6'
Extensions directory '/Users/shaharmor/.azure/cliextensions'

Python (Darwin) 3.6.4 (default, Dec 19 2017, 15:24:51)
[GCC 4.2.1 Compatible Apple LLVM 9.0.0 (clang-900.0.39.2)]
```