[ad-hoc] [1] Add validation to makeFilterVolume

[debnil]

We want to add validation to the config (of type *VolumeFilterConfig in makeFilterVolume in plugins/volumeFilter.go. This validation should include checking that exactly one asset cap is non-nil; the mode is valid; and optional accountIDs and market IDs are both non-nil.

Once the above is added, a test case should be added to TestMakeFilterVolume within volumeFilter_test.go. This test case should confirm that if an invalid configuration is constructed, an error is thrown by makeFilterVolume.

[debnil]

The requirements for this have been amended - market IDs and account IDs can be nil, and the action must be valid. So we only need to validate that exactly one cap is non-nil, and the mode and action are valid.