Fix: Check if plugin file exists before loading

[druid-infra]

Problem

When druid-cli tries to load a plugin that doesn't exist (e.g., druid_rcon), it fails with a confusing error:

Error in Daemon Startup: unable to generate a checksum for the plugin ./druid_rcon: open ./druid_rcon: no such file or directory

This happens even when plugins are optional and the --allow-plugin-errors flag should handle it gracefully.

Root Cause

The LoadGoPlugin function tries to generate a checksum for the plugin file without first verifying it exists. The checksum generation fails with an opaque error that doesn't clearly indicate the file is missing.

Solution

• Add explicit file existence check before attempting to load the plugin
• Return a clearer error message that shows all searched paths
• This allows the --allow-plugin-errors flag to work properly
• Makes debugging easier by showing exactly where druid looked for the plugin

Example Error (Before)

unable to generate a checksum for the plugin ./druid_rcon: open ./druid_rcon: no such file or directory

Example Error (After)

plugin file not found: ./druid_rcon (looked in /usr/local/bin/druid_rcon and ./druid_rcon)

Testing

Reproduced in CI environment where plugin files were missing. After this fix, the error message is clearer and the --allow-plugin-errors flag can properly handle the situation.

Related issue discovered in: highcard-dev/scrolls#12

[druid-infra]

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