Tolerate adapted body delivery failures in REQMOD request satisfaction mode.

Tolerate adapted body delivery failures in REQMOD request satisfaction mode.

Without these changes, Squid may assert if an ICAP or eCAP service fails
while delivering response body in REQMOD:
    assertion failed: client_side.cc:1438: "rep"

Other assertions may be possible as well, because we were trying to
serve a freshly built, HttpReply-free error response while already
writing the REQMOD "request satisfaction" response. The assertion
probably depends on that writing stage (wrote nothing yet, wrote
headers, wrote some body, etc).

Polished ERR_DETAIL constants to distinguish the special "request
satisfaction" case and to remove ICAP-specific labels from general
adaptation code.