ziti-edge-tunnel: enroll should not overwrite json file on failure

[dovholuknf]

right now issuing ziti-edge-tunnel enroll -j jwt.file -i identity.file twice will work the first time, then fail the second time. if you make this mistake you MUST recreate your identity file. it would be much better to not overwrite this file if the command is run twice

[qrkourier]

Similarly, enroll token is used if output file is not writeable.

[qrkourier]

@dovholuknf Is this a good first issue?

[qrkourier]

This issue is valid and reproducible in 0.19.6.

❯ ./ziti-edge-tunnel version
v0.19.6

❯ ./ziti-edge-tunnel enroll -j ./client4.jwt -i ./client4.json

❯ ll client4.j*
-rw-rw-r-- 1 kbingham kbingham 955 Jul 28 09:44 client4.jwt
-rw-rw-r-- 1 kbingham kbingham 24K Jul 28 09:45 client4.json

❯ ./ziti-edge-tunnel enroll -j ./client4.jwt -i ./client4.json
[        0.320]   ERROR ziti-sdk:ziti_enroll.c:228 enroll_cb() failed to enroll with controller: https://7ce7e424-6a92-4ff2-9459-ebbba32346fa.production.netfoundry.io:443 INVALID_ENROLLMENT_TOKEN (The supplied token is not valid)
[        0.320]   ERROR ziti-edge-tunnel:ziti-edge-tunnel.c:1973 enroll_cb() enrollment failed: INVALID_ENROLLMENT_TOKEN(-3)

❯ ll client4.j*                                               
-rw-rw-r-- 1 kbingham kbingham 955 Jul 28 09:44 client4.jwt
-rw-rw-r-- 1 kbingham kbingham   0 Jul 28 09:45 client4.json