Introduce validation and error handling for validation preview

app/main/forms.py

@@ -769,7 +769,10 @@ class ServiceUpdateEmailBranding(StripWhitespaceForm):
 class PDFUploadForm(StripWhitespaceForm):
     file = FileField_wtf(
         'Upload a letter in PDF format to check if it fits in the printable area',
-        validators=[FileAllowed(['pdf'], 'PDF documents only!')]
+        validators=[
+            FileAllowed(['pdf'], 'PDF documents only!'),
+            DataRequired(message="You need to upload a file to submit")
+        ]
     )
 
 

app/main/views/platform_admin.py

@@ -248,13 +248,19 @@ def platform_admin_letter_validation_preview():
     message, pages, result = None, [], None
     form = PDFUploadForm()
 
-    if request.method == "POST":
+    if form.validate_on_submit():
         pdf_file = form.file.data
-        response = validate_letter(pdf_file)
-        if response.status_code == 200:
-            pages=response.json()["pages"]
-            message = response.json()["message"]
-            result = response.json()["result"]
+        try:
+            response = validate_letter(pdf_file)
+            if response.status_code == 200:
+                pages=response.json()["pages"]
+                message = response.json()["message"]
+                result = response.json()["result"]
+        except HTTPError as error:
+            if error.status_code == 400:
+                flash("Something was wrong with the file you tried to upload. Please upload a valid PDF file.")
+            else:
+                raise e
 
     return render_template(
         'views/platform-admin/letter-validation-preview.html',