Respond gracefully when benchmarks not set up on baseline

[epatters]

Thanks for this useful package! I am currently setting up both PkgBenchmark and BenchmarkCI in the same branch of my repo. As a result, the file benchmark/benchmarks.jl does not exist on the master branch. This causes the baseline run of benchmarks to fail on the pending PR.

I had to dig into the BenchmarkCI source code to figure out what was going wrong. It would be nice if a more informative error, or even just a warning, was raised when the baseline branch is not set up for benchmarks.

[tkf]

Yeah, this has been in my (although rather low-priority) TODO #37 but let's track it here.

[epatters]

Whoops, didn't see that. Thanks.

[filchristou]

I guess we can close this :) ?