Log function error on failure

[carlzogh]

Issue #, if available:
#411

Description of changes:
Log serialized function error when function invocation fails. This change only affects the non-RPC path where the library is used as a standalone Runtime Interface Client.

The error message format is not the same as the go1.x runtime, as this one is serialized and includes a timestamp.

Testing:

1. Built the change and tested on provided.al2 runtime, managed to see error in invocation logs:

START RequestId: a3dc012e-ab86-472e-b7ba-da56bfa92e5d Version: $LATEST
2021/12/08 12:33:00 {"errorMessage":"error in CW test","errorType":"errorString"}
END RequestId: a3dc012e-ab86-472e-b7ba-da56bfa92e5d
REPORT RequestId: a3dc012e-ab86-472e-b7ba-da56bfa92e5d  Init Duration: 3.71 ms  Duration: 39.46 ms      Billed Duration: 40 ms  Memory Size: 3008 MB    Max Memory Used: 3008 MB 

whereas before this change:

START RequestId: 1553ae39-0f8a-4172-86fb-3db7090619e7 Version: $LATEST
END RequestId: 1553ae39-0f8a-4172-86fb-3db7090619e7
REPORT RequestId: 1553ae39-0f8a-4172-86fb-3db7090619e7  Init Duration: 0.60 ms  Duration: 11.00 ms      Billed Duration: 12 ms  Memory Size: 3008 MB    Max Memory Used: 3008 MB        

2. Ran on go1.x runtime and did not see a change in logged messages:

START RequestId: 75337e13-e8d0-48c9-b40b-74c2b7394622 Version: $LATEST
error in CW test: errorString
null
END RequestId: 75337e13-e8d0-48c9-b40b-74c2b7394622
REPORT RequestId: 75337e13-e8d0-48c9-b40b-74c2b7394622  Init Duration: 0.36 ms  Duration: 23.98 ms      Billed Duration: 24 ms  Memory Size: 3008 MB    Max Memory Used: 3008 MB     

3. Rebuilt go1.x internally with this change, did not see a change in logged message either:

START RequestId: 918788fb-1715-4961-ad3f-cc068c120803 Version: $LATEST
error in CW test: errorString
null
END RequestId: 918788fb-1715-4961-ad3f-cc068c120803
REPORT RequestId: 918788fb-1715-4961-ad3f-cc068c120803  Init Duration: 0.47 ms  Duration: 29.90 ms      Billed Duration: 30 ms  Memory Size: 3008 MB    Max Memory Used: 3008 MB       

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