Buffet: utility function tweaks

Various tweakes to helper functions and classes, split off from
a larger CL that required them.

1. Added Error::AddToPrintf to help add formatted error messages.
2. Added Error::GetFirstError to get the innermost error occurred.
3. Added string_utils::ToString and swept code using std::to_string
   in order to ensure we format doubles correctly (using %g instead
   of %f format specifier) and bool values (using "true"/"false"
   instead of 1/0).
4. Fixed C-style cast in http_utils.h and using static_cast now.
5. Fixed a few linter warnings. Also since the linter was updated
   there is no reason to have some NOLINT since many C++11 features
   are now recognized properly by cpplint.

BUG=None
TEST=All unit tests pass.

Change-Id: I208ffaa3f0ec0a5ff78bf9e8151e784ec8cd77e2
Reviewed-on: https://chromium-review.googlesource.com/202962
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>

buffet/http_connection_fake.cc

diff --git a/buffet/http_connection_fake.cc b/buffet/http_connection_fake.cc
index 87fbf23..383f6d4 100644
--- a/buffet/http_connection_fake.cc
+++ b/buffet/http_connection_fake.cc
@@ -37,7 +37,7 @@
 
 bool Connection::FinishRequest(ErrorPtr* error) {
   request_.AddHeaders({{request_header::kContentLength,
-                      std::to_string(request_.GetData().size())}});
+                      string_utils::ToString(request_.GetData().size())}});
   fake::Transport* transport = static_cast<fake::Transport*>(transport_.get());
   CHECK(transport) << "Expecting a fake transport";
   auto handler = transport->GetHandler(request_.GetURL(), request_.GetMethod());