XrdTpc: Always populate error buffer with messages.

Even when there's an internal logic error, make sure the Write
method populates the error message - if nothing else, will be useful
for debugging.

src/XrdTpc/XrdTpcStream.cc

@@ -67,10 +67,14 @@ Stream::Write(off_t offset, const char *buf, size_t size, bool force)
     m_log.Emsg("Stream::Write", ss.str().c_str());
     DumpBuffers();
 */
-    if (!m_open_for_write) return SFS_ERROR;
+    if (!m_open_for_write) {
+        if (!m_error_buf.size()) {m_error_buf = "Logic error: writing to a buffer not opened for write";}
+        return SFS_ERROR;
+    }
     size_t bytes_accepted = 0;
     int retval = size;
     if (offset < m_offset) {
+        if (!m_error_buf.size()) {m_error_buf = "Logic error: writing to a prior offset";}
         return SFS_ERROR;
     }
     // If this is write is appending to the stream and
@@ -107,6 +111,7 @@ Stream::Write(off_t offset, const char *buf, size_t size, bool force)
             // going to force a flush even if things are not MB-aligned.
             int retval2 = (*entry_iter)->Write(*this, size == 0);
             if (retval2 == SFS_ERROR) {
+                if (!m_error_buf.size()) {m_error_buf = "Unknown filesystem write failure.";}
                 return retval2;
             }
             buffer_was_written |= retval2 > 0;
@@ -133,10 +138,13 @@ Stream::Write(off_t offset, const char *buf, size_t size, bool force)
     m_avail_count = avail_count;
 
     if (bytes_accepted != size && size) {  // No place for this data in allocated buffers
-        if (!avail_entry) {  // No available buffers to allocate.
+        if (!avail_entry) {  // No available buffers to allocate; logic error, should not happen.
+            DumpBuffers();
+            m_error_buf = "No empty buffers available to place unordered data.";
             return SFS_ERROR;
         }
         if (avail_entry->Accept(offset + bytes_accepted, buf, size - bytes_accepted) != size - bytes_accepted) {  // Empty buffer cannot accept?!?
+            m_error_buf = "Empty re-ordering buffer was unable to to accept data; internal logic error.";
             return SFS_ERROR;
         }
         m_avail_count --;
@@ -177,6 +185,11 @@ void
 Stream::DumpBuffers() const
 {
     m_log.Emsg("Stream::DumpBuffers", "Beginning dump of stream buffers.");
+    {
+        std::stringstream ss;
+        ss << "Stream offset: " << m_offset;
+        m_log.Emsg("Stream::DumpBuffers", ss.str().c_str());
+    }
     size_t idx = 0;
     for (std::vector<Entry*>::const_iterator entry_iter = m_buffers.begin();
          entry_iter!= m_buffers.end();