Improve error message for invalid template when registering build

[Code42Cate]

When starting a build with an invalid alias you would get:

Error requesting template build: [400] bad request: Invalid alias:

because alias is always an empty string if validation fails. Now, we return the original env ID. This makes the error handling also consistent with the other places where id.CleanEnvID is used

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Note

Uses the original input alias in the invalid-alias error message in register_build.go instead of the cleaned (often empty) value.

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