Hi Andrew,
Inner error comes with SAP's ES Workplace Demo services. For example
for a request
https://sapes1.sapdevcenter.com/sap/opu/odata/sap/ZCD204_EPM_DEMO_SRV/Products('1')?$format=json
I get response as
{"error":{"code":"SEPM_BOR_MESSAGES/003","message":{"lang":"en","value":"Consumer problem: Invalid product Id 1"},"innererror":{"transactionid":"52172C7D443D19C6E1000000AC140C37","errordetails":[{"code":"SEPM_BOR_MESSAGES/003","message":"Consumer problem: Invalid product Id 1","propertyref":"","severity":"error"},{"code":"/IWBEP/CX_MGW_BUSI_EXCEPTION","message":"No exception text available","propertyref":"","severity":"error"}]}}}
So to answer your question, json also provides Inner error. (may be not with your current SP)
Thanks
Krishna