Clarify "missing_placeholder_in_template" error message

[JanCizmar]

When the CLI exports keys that belong to more than one namespace, it currently throws the missing_placeholder_in_template error if fileStructureTemplate lacks the {namespace} placeholder. The message, however, does not explain why this is required, leading to confusion.

Steps to Reproduce

Configure fileStructureTemplate without the {namespace} placeholder, e.g.

fileStructureTemplate: "locales/{language}.json"

Export namespaces common and admin together using the CLI.

Current Behavior

The CLI fails with the error code missing_placeholder_in_template, but the message does not clarify the root cause or how to fix it.

Expected Behavior

The error message should:

• Explicitly state that multiple namespaces were detected.
• Explain that writing all namespaces to a single file leads to key collisions and unpredictable results.
• Point out that the purpose of namespaces is to separate content into distinct files.
• Provide an actionable fix (i.e., add {namespace} to the template or export one namespace at a time).

Proposed Error Message

Error: Your export involves multiple namespaces (e.g. "common", "admin"), but
`fileStructureTemplate` does not contain the `{namespace}` placeholder.

Without `{namespace}`, the CLI would merge all namespaces into the same file,
causing key conflicts and unpredictable overrides.  
→ Add `{namespace}` to `fileStructureTemplate` (e.g. "locales/{namespace}/{language}.json")  
  or export a single namespace at a time.

Additional Context

Namespaces exist precisely so that translations can be split into separate files; a single‑file export defeats that purpose.

Because there is no deterministic way to resolve key collisions across namespaces, the CLI must enforce this constraint.