More sane error reporting

[elliotchance]

Up until this point most errors result in a panic. This is useful when early stages debugging but it would be extremely painful for anyone trying to properly use the tool if every tiny infringements caused the whole process to stop.

This PR will print errors and warnings as comments at the top of the file, but try to continue sensibly so that it will produce the best Go it can even if that turns out to be not completely correct.

I am testing this on the whole amalgamated SQLite3 source code. Obviously c2go is not at all mature enough to transpire this. However, if i can get it to spit out lots of warnings and at least produce some Go it will be a major step in the right direction.

I have a feeling this PR will turn out to have a great lot of changes so it's likely it will have around for a while, then get it's own release.

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This change is 

[codecov]

Codecov Report

Merging #112 into master will decrease coverage by 3.85%.
The diff coverage is 40.87%.

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
- Coverage   69.11%   65.25%   -3.86%     
==========================================
  Files         120      120              
  Lines        3215     3526     +311     
==========================================
+ Hits         2222     2301      +79     
- Misses        910     1119     +209     
- Partials       83      106      +23

Impacted Files	Coverage Δ
program/struct.go	78.57% <0%> (-6.05%)	⬇️
transpiler/operators.go	88.19% <0%> (-2.52%)	⬇️
ast/ast.go	39.32% <15%> (-24.88%)	⬇️
transpiler/transpiler.go	88.12% <33.33%> (-0.63%)	⬇️
util/goast.go	66.66% <33.33%> (+4.16%)	⬆️
transpiler/call.go	79.61% <42.85%> (-14.06%)	⬇️
transpiler/unary.go	67.04% <50%> (-6.2%)	⬇️
transpiler/switch.go	78.49% <50%> (-1.51%)	⬇️
transpiler/branch.go	67.47% <55.55%> (-3.17%)	⬇️
types/resolve.go	73.33% <64.28%> (-10.99%)	⬇️
... and 8 more

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