add try/catch to all callback functions (never unwind through them)

[StefanKarpinski]

This may help #196. The issue there may be that callback functions raise Julia errors sometimes (they shouldn't but stuff happens, ya know?). This wraps each one in a try/catch block and logs an "unkonwn error" message if that happens and returns from the callback with an appropriate return code to indicate error. The libcurl docs aren't super clear on these and this is hard to test so hopefully I got it right, but we'll see. Should be better.

[DilumAluthge]

Is this backportable?

[StefanKarpinski]

Probably not cleanly, but the same thing could maybe be done on the old branch.

[codecov]

Codecov Report

Merging #206 (7207eaf) into master (0733701) will decrease coverage by 2.56%.
The diff coverage is 77.66%.

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
- Coverage   92.22%   89.65%   -2.57%     
==========================================
  Files           5        5              
  Lines         527      551      +24     
==========================================
+ Hits          486      494       +8     
- Misses         41       57      +16     

Impacted Files	Coverage Δ
src/Curl/Easy.jl	91.72% <73.21%> (-4.05%)	⬇️
src/Curl/Multi.jl	91.33% <82.97%> (-2.88%)	⬇️

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