Completions do not appear if command table dump encounters an exception

[tjprescott]

When az interactive starts, if an exception is encountered during the command table dumping, the table is not cached nor updated, preventing static and dynamic completions from working. We should aim to make this process more fault-tolerant.

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Environment summary

Install Method (e.g. pip, interactive script, apt-get, Docker, MSI, edge build) / CLI version (az --version) / OS version / Shell Type (e.g. bash, cmd.exe, Bash on Windows)

azure-cli (2.0.27)

acr (2.0.21)
acs (2.0.26)
advisor (0.1.2)
appservice (0.1.26)
backup (1.0.6)
batch (3.1.10)
batchai (0.1.5)
billing (0.1.7)
cdn (0.0.13)
cloud (2.0.12)
cognitiveservices (0.1.10)
configure (2.0.14)
consumption (0.2.1)
container (0.1.18)
core (2.0.27)
cosmosdb (0.1.19)
dev-tools (0.1.1)
dla (0.0.18)
dls (0.0.19)
eventgrid (0.1.10)
extension (0.0.9)
feedback (2.1.0)
find (0.2.8)
interactive (0.3.16)
iot (0.1.17)
keyvault (2.0.18)
lab (0.0.17)
monitor (0.1.2)
network (2.0.23)
nspkg (3.0.1)
profile (2.0.19)
rdbms (0.0.12)
redis (0.2.11)
reservations (0.1.1)
resource (2.0.23)
role (2.0.19)
servicefabric (0.0.10)
sql (2.0.21)
storage (2.0.25)
testsdk (0.1.0)
vm (2.0.26)

Extensions:
subscription (0.1.0)

Python location 'C:\Users\trpresco\Documents\github\azure-cli\env\Scripts\python.exe'
Extensions directory 'C:\Users\trpresco\.azure\cliextensions'

Python (Windows) 3.6.1 (v3.6.1:69c0db5, Mar 21 2017, 17:54:52) [MSC v.1900 32 bit (Intel)]