Fix: ctf-text should link on libbabeltrace-ctf

Refs: #410

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Fix: add missing error return when create_event_definitions fails

Also add more verbose error printout to help identifying the culprit of
metadata issues.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Print warning to stderr if a ctf-text read is attempted
Fix two compiler warnings

Clang reported an uninitialized variable, ret, in ctf.c and an always-false
condition (checking if an unsigned value < 0) caused by a type mismatch in
ctf-visitor-generate-io-struct.c.
Remove unused array in bt_context_add_traces_recursive
Add bt_stream_pos to bt_trace_descriptor

This is intended to provide a generic way to access the bt_stream_pos API
without having to make assumptions about the output format's descriptor layout.
Fix code relying on the output format being ctf-text