Fix adb sync/pull/push error reporting.

I fixed the server side to send detailed error reports, but the client-side
code for pull was broken because it had already read the "FAIL" header before
calling the general error reporting code that expects to be able to read
the header. This meant we'd always report that we failed to read the failure
message.

Also add a couple of missing "\n"s, make sure every error message is prefixed
by "adb: ", and remove a useless path length check that would silently ignore
over-long paths rather than relying on SendRequest to detect and report the
problem.

Bug: http://b/6205106
Change-Id: I23862ececf03b761115ffa3f7725b7e1cecb48c7