Improve generator process creation error

[cd-work]

This patch changes the error generated during lockfile generation when the ecosystem's package manager subprocess could not be spawned successfully.

Instead of printing the full debug formatting of the command, just the program and its argument are printed.

Closes #1084.

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New error message example:

Generating lockfile for manifest "./requirements.txt" using Pip…
❗ Error: failed to spawn command "pip-compile": No such file or directory (os error 2)

[maxrake]

The changes here were applied to the pip-tools branch (#1075) and tested locally. The results are as advertised.