fix(exec): use client err for substitution failures and delay PlanStep

[ecrupper]

Any substitution errors were failing silently and not being relayed to the user in any fashion. Further, because PlanStep was executed before the substitution, the step is marked as running and is not properly cleaned up.

By using c.err, the build will bomb out expectedly if there's a container config breaking error and will also set the upcoming step as skipped instead of success when it's all said and done.

Example Pipeline:

version: "1"

steps:
  - name: sleepy
    image: hub.docker.target.com/alpine:latest
    commands:
      - sleep 5

  - name: mess up
    image: hub.docker.target.com/alpine:latest
    parameters:
      tags:
        - ${VELA_BUILD_NUMBER:0:8} // FAILED SUBSTITUTION (number cannot be indexed :0:8)
    commands:
      - echo "This is a test"

BEFORE:

AFTER:

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.67%. Comparing base (38835a1) to head (062ab61).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
executor/linux/build.go	40.00%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #701   +/-   ##
=======================================
  Coverage   52.67%   52.67%           
=======================================
  Files         130      130           
  Lines        5635     5635           
=======================================
  Hits         2968     2968           
  Misses       2438     2438           
  Partials      229      229           

Files with missing lines	Coverage Δ
executor/linux/stage.go	73.78% <100.00%> (ø)
executor/linux/build.go	67.36% <40.00%> (ø)

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