Use "danger" alert class to communicate that an error occurred

Updates the alert class to make it clearer to users than an error has occurred
where the previous colors indicated a warning (something less actionable).

index.html

@@ -32,7 +32,7 @@ <h4>Instructions:</h4>
 
     <div class="row mb-5 border-top border-bottom pt-3 pb-3">
       <div class="col-8">
-        <div class="alert alert-warning" role="alert" id="dataFormFieldAlert" hidden>
+        <div class="alert alert-danger" role="alert" id="dataFormFieldAlert" hidden>
           <strong>Error reading the data URL!</strong>
           Please review the URL below or check the contents of the datafile by reviewing the
           <a href="https://dms-view.github.io/docs/dataupload/" target="_blank">documentation</a>.
@@ -49,7 +49,7 @@ <h4>Instructions:</h4>
         <div id="logo_plot" class="row"></div>
       </div>
       <div class="col-4">
-        <div class="alert alert-warning" role="alert" id="proteinFormFieldAlert" hidden>
+        <div class="alert alert-danger" role="alert" id="proteinFormFieldAlert" hidden>
           <strong>Error reading the protein structure URL!</strong>
           Please review the URL below or check the contents of the protein structure by reviewing the
           <a href="https://dms-view.github.io/docs/dataupload/" target="_blank">documentation</a>.
@@ -67,7 +67,7 @@ <h4>Instructions:</h4>
 
     <div id="description" class="row mt-3">
       <div class="col-12">
-        <div class="alert alert-warning" role="alert" id="markdownFormFieldAlert" hidden>
+        <div class="alert alert-danger" role="alert" id="markdownFormFieldAlert" hidden>
           <strong>Error reading the description URL!</strong>
           Please review the URL below or check the contents of the description markdown by reviewing the
           <a href="https://dms-view.github.io/docs/dataupload/" target="_blank">documentation</a>.