fix backtraces in Jsonrpc process
fix backtraces in Jsonrpc process

Summary:
`Printexc.to_string e` overwrites backtraces, so calling it before `Printexc.get_backtrace ()` blows away the trace you're trying to get! `Exception` exists solely to solve this issue, so we should use it...

before:
```
[Client fatal] End_of_file
Raised at file "string.ml", line 115, characters 19-34
Called from file "src/sexp.ml", line 113, characters 13-47
```

after:
```
[Client fatal] End_of_file
Raised at file "/home/mroch/flow/src/hack_forked/utils/jsonrpc/jsonrpc.ml", line 233, characters 6-172
Called from file "src/core/lwt.ml", line 2100, characters 23-28
Called from file "src/core/lwt.ml", line 1211, characters 10-18
Called from file "src/core/lwt.ml", line 1277, characters 17-21
Called from file "src/core/lwt.ml", line 1313, characters 4-103
Called from file "src/core/lwt.ml", line 1942, characters 12-71
Called from file "src/core/lwt.ml", line 1211, characters 10-18
Called from file "src/core/lwt.ml", line 1277, characters 17-21
Called from file "src/core/lwt.ml", line 1313, characters 4-103
Called from file "src/core/lwt.ml", line 1888, characters 12-55
Called from file "src/core/lwt.ml", line 1211, characters 10-18
Called from file "src/core/lwt.ml", line 1277, characters 17-21
Called from file "src/core/lwt.ml", line 1313, characters 4-103
Called from file "src/core/lwt.ml", line 1878, characters 12-72
Called from file "src/core/lwt.ml", line 1211, characters 10-18
Called from file "src/core/lwt.ml", line 1277, characters 17-21
Called from file "src/core/lwt.ml", line 1313, characters 4-103
Called from file "src/core/lwt.ml", line 1878, characters 12-72
Called from file "src/core/lwt.ml", line 1211, characters 10-18
Called from file "src/core/lwt.ml", line 1277, characters 17-21
Called from file "src/core/lwt.ml", line 1313, characters 4-103
Called from file "src/core/lwt.ml", line 1878, characters 12-72
Called from file "src/core/lwt.ml", line 1211, characters 10-18
Called from file "src/core/lwt.ml", line 1277, characters 17-21
Called from file "src/core/lwt.ml", line 1313, characters 4-103
Called from file "src/core/lwt.ml", line 1387, characters 37-76
Called from file "array.ml", line 93, characters 31-48
Called from file "src/core/lwt_sequence.ml", line 132, characters 31-47
Called from file "list.ml", line 110, characters 12-15
Called from file "src/unix/lwt_engine.ml", line 359, characters 4-63
Called from file "src/unix/lwt_main.ml", line 41, characters 6-49
Called from file "src/unix/lwt_main.ml", line 118, characters 8-13
Called from file "/home/mroch/flow/src/common/lwt/lwtInit.ml", line 129, characters 4-135
Called from file "/home/mroch/flow/src/commands/commandUtils.ml", line 13, characters 4-32
Called from file "/home/mroch/flow/src/flow.ml", line 110, characters 4-21
```

Reviewed By: gabelevi

Differential Revision: D28816941

fbshipit-source-id: dbcef82f735c11be791eef88fc2674cbb2906693

facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow