Improve missing invoc img err

[chris-crone]

- What I did Add a better error message for when Docker App cannot find the base invocation image.

- How I did it

- How to verify it Build a version of docker-app without a corresponding base invocation image on the Hub. Run docker-app inspect examples/hello-world/hello-world.dockerapp. Expect to see:

unable to resolve Docker App base image: docker/cnab-app-base:v0.6.0-204-g4409f471e5-dirty

- Description for the changelog None

- A picture of a cute animal (not mandatory but encouraged)

[codecov[bot]]

Codecov Report

Merging #500 into master will decrease coverage by 1.71%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #500      +/-   ##
==========================================
- Coverage   71.11%   69.39%   -1.72%     
==========================================
  Files          51       51              
  Lines        2849     2745     -104     
==========================================
- Hits         2026     1905     -121     
- Misses        594      595       +1     
- Partials      229      245      +16

Impacted Files	Coverage Δ
internal/packager/packing.go	21.17% <ø> (ø)	:arrow_up:
internal/commands/bundle.go	63.79% <0%> (-2.28%)	:arrow_down:
pkg/yatee/yatee.go	68.5% <0%> (-11.13%)	:arrow_down:
types/parameters/parameters.go	85.71% <0%> (-11.12%)	:arrow_down:
internal/commands/root.go	100% <0%> (ø)	:arrow_up:
internal/commands/completion.go	48.48% <0%> (+2.77%)	:arrow_up:
internal/commands/push.go	38.6% <0%> (+3.41%)	:arrow_up:

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[silvin-lubecki]

PTAL @simonferquel