improve missing end tag errors

[MartijnR]

Currently the error message is utterly useless (in ODK Validate it is much better):

Errors:
Unexpected close tag Line: 94, Column: 38, Char: >

e.g. for the following <instance> (PROCEDURE is not closed):

    <instance>
        <Last_group_does_not_display id="PSS" version="1">
          <PROCEDURE>
            <DOP/>
            <SURG/>
            <PROC_NOTE/>
            <PROC_GRID jr:template="">
              <PROC/>
              <LATER/>
              <TMLOC_BLANK/>
              <PMLOC/>
              <SLNDLOC/>
              <CALC_LOC/>
            </PROC_GRID>
            <CALC_SPEC_TTL1/>
            <CALC_SHOW1/>
            <CALC_SHOW2/>
            <CALC_SPEC_TTL2/>
          <SPEC>
            <GPT>
              <TUMTYP/>
              <PM_TUMLENGTH/>
              <PM_TUMWIDTH/>
              <PM_TUMUNITS/>
              <PM_LGMLENGTH/>
              <PM_LGMWIDTH/>
              <PM_LGMUNITS/>
              <PM_MGMLENGTH/>
              <PM_MGMWIDTH/>
              <PM_MGMUNITS/>
              <PM_LENGTH/>
              <PM_WIDTH/>
              <PM_UNITS/>
              <ENTRIES>
                <PM jr:template="">
                  <PM_SPECIM/>
                  <PM_SPECIMHIST/>
                  <PM_GSS/>
                  <PM_MEDIALAT/>
                  <PM_SLICEMETH/>
                  <PM_ANTDEEP/>
                  <PM_SLICEMETH2/>
                  <PM_SUPINF/>
                  <PM_SLICEMETH3/>
                  <PM_SPECSLICE/>
                  <PM_TOTSLICE/>
                  <PM_TOTSLICEMETH/>
                  <PM_SLICENOTE/>
                  <PM_SLICEPL/>
                  <PM_SLICENUM/>
                  <PM_BLOCKNUM/>
                  <PM_SLICEPL2/>
                  <PM_SLICENUM2/>
                  <PM_BLOCKNUM2/>
                  <PM_PERFLAB/>
                  <PM_LYMNTISS/>
                  <PM_NOTE/>
                </PM>
                <TM jr:template="">
                  <TM_SPECIM/>
                  <TM_SPECIMHIST/>
                  <TM_GSS/>
                  <TM_MEDIALAT/>
                  <TM_SLICEMETH/>
                  <TM_ANTDEEP/>
                  <TM_SLICEMETH2/>
                  <TM_SUPINF/>
                  <TM_SLICEMETH3/>
                  <TM_TOTSLICE/>
                  <TM_TOTSLICEMETH/>
                  <TM_SLICENOTE/>
                  <TM_SLICEPL/>
                  <TM_SLICENUM/>
                  <TM_BLOCKNUM/>
                  <TM_SLICEPL2/>
                  <TM_SLICENUM2/>
                  <TM_BLOCKNUM2/>
                  <TM_PERFLAB/>
                  <TM_LYMNTISS/>
                  <TM_NOTE/>
                </TM>
              </ENTRIES>
            </GPT>
          </SPEC>
          <LYMPHNODES>
            <LYMNDISS/>
          </LYMPHNODES>
          <meta>
            <instanceID/>
          </meta>
        </Last_group_does_not_display>
      </instance>