Bring focus to errors in the log

In addition to reporting failures first as described in #493, I'd like to see the relevant part of the log first, to avoid having to scroll through the huge log to find the interesting lines. More precisely, this is what I'm most interesting in seeing:

#=== ERROR while installing jane-street-tests.113.33.03 =======================#
# opam-version         1.2.2 (58ef3b8213100953848d362f7120a30356d7f77d)
# os                   linux
# command              make
# path                 /home/opam/.opam/4.02.3/build/jane-street-tests.113.33.03
# compiler             4.02.3
# exit-code            2
# env-file             /home/opam/.opam/4.02.3/build/jane-street-tests.113.33.03/jane-street-tests-29-b5c6f4.env
# stdout-file          /home/opam/.opam/4.02.3/build/jane-street-tests.113.33.03/jane-street-tests-29-b5c6f4.out
# stderr-file          /home/opam/.opam/4.02.3/build/jane-street-tests.113.33.03/jane-street-tests-29-b5c6f4.err
### stdout ###
# [...]
# -@@ -7,4 +7,4 @@
# - 
# - let%expect_test "expect test2" =
# -   print_string "bar";
# --  [%expect {|XXX|}]
# -+  [%expect {|bar|}]
# -[1]
# ../../makefiles/Makefile.simple:10: recipe for target 'test' failed
# make[1]: Leaving directory '/home/opam/.opam/4.02.3/build/jane-street-tests.113.33.03/test-inline-tests/with-oasis-method1'
# Makefile:9: recipe for target 'tests' failed
### stderr ###
# make[1]: *** [test] Error 1
# make: *** [tests] Error 2

Moreover, opam only shows the end of the build log and as a result we often miss valuable information that would help debugging. It would be great if the various XXX-file listed in the previous output were made easily available. For instance it would be awesome if we could just click on them to access their contents.

[talex5]

I guess this is really an issue for mirage-ci (@avsm).

Building with ENV OPAMERRLOGLEN=0 should avoid opam truncating the output, and should avoid the need to do everything in verbose mode.