Report filename:line on runtime errors (fix #752)

With this patch, jq run-time errors printed to stderr will contain the
filename and line of the offending input.

But note that the JSON text parser does (yet) print the input filename
on parse error.  A jq run-time error is a jq program error, not
including syntax errors nor JSON text input format errors.

Examples:

With stdin and multiple lines:

    $ printf '{"a":43}\n{"a":{"b":66}}\n' | ./jq '.a+1'
    44
    jq: error (at stdin:2): object and number cannot be added

With multiple files:

    $ printf '{"a":43}' > 1.json
    $ printf '{"a":"hello"}\n' > 2.json
    $ printf '{"a":{"b":66}}\n' > 3.json
    $ ./jq '[.a]|@tsv' 1.json 2.json 3.json
    "43"
    "hello"
    jq: error (at 3.json:1): object is not valid in a csv row

With very long lines (spanning multiple `fgets` calls):

    $ (  printf '{"a":43}\n' ;
         printf '{"a":{"b":[' ; seq 10000 | paste -d, -s | tr -d '\n' ;
         printf ']}}\n' ;
         printf '{"a":"hello"}\n' ) | ./jq '[.a] | @tsv'
    "43"
    jq: error (at stdin:2): object is not valid in a csv row
    "hello"

With raw input:

    $ seq 1000 | ./jq --raw-input 'select(.=="700") | . + 10'
    jq: error (at stdin:700): string and number cannot be added

Caveat:
The reported line will be the last line of the (valid) parsed JSON data.
Example:

    $ printf '{\n"a":\n"hello"\n\n\n}\n' | ./jq '.a+4'
    jq: error (at stdin:6): string and number cannot be added

main.c

@@ -132,13 +132,17 @@ static int process(jq_state *jq, jv value, int flags, int dumpopts) {
   if (jv_invalid_has_msg(jv_copy(result))) {
     // Uncaught jq exception
     jv msg = jv_invalid_get_msg(jv_copy(result));
+    jv input_pos = jq_util_input_get_position(jq);
     if (jv_get_kind(msg) == JV_KIND_STRING) {
-      fprintf(stderr, "jq: error: %s\n", jv_string_value(msg));
+      fprintf(stderr, "jq: error (at %s): %s\n",
+              jv_string_value(input_pos), jv_string_value(msg));
     } else {
       msg = jv_dump_string(msg, 0);
-      fprintf(stderr, "jq: error (not a string): %s\n", jv_string_value(msg));
+      fprintf(stderr, "jq: error (at %s) (not a string): %s\n",
+              jv_string_value(input_pos), jv_string_value(msg));
     }
     ret = 5;
+    jv_free(input_pos);
     jv_free(msg);
   }
   jv_free(result);