More descriptive error in io.LoadSequenceList()

The reason are potentially huge fasta files for which debugging is super
hard when you're just told: something is wrong...

modules/io/src/seq/fasta_io_handler.cc

@@ -111,7 +111,7 @@ void FastaIOHandler::Import(seq::SequenceList& aln,
         aln.AddSequence(seq);          
         seq_count+=1;
       } catch (seq::InvalidSequence& e) {
-        throw e;
+        throw seq::InvalidSequence("Failed for sequence with name " + name + ": " + e.what());
       }
     } else {
       throw IOException("Bad FASTA file: Sequence is empty.");

modules/seq/base/src/invalid_sequence.hh

@@ -31,6 +31,9 @@ public:
   InvalidSequence()
     : Error("Sequence String contains illegal characters. Must "
                          "be one of [A-Za-z?-].") { }
+
+  InvalidSequence(const String& custom_msg)
+    : Error(custom_msg) {}
 };
 
 class DLLEXPORT InvalidAlignment : public Error {