This is caused by an item with invalid metadata (the crash is occurring when it tries to get the icon and they did not put a bounds check in; I know this because I have MCP for 1.7.10 and "func_149691_a" corresponds to Block.getIcon), and it is in your inventory since it references "GuiIngame". You can fix this by opening level.dat with NBTExplorer and looking at the player inventory (under Player, Inventory); if it is crashing as soon as you load the world it will be in your hot bar, slots 0-8. Since you are in 1.7.10 items use numerical IDs so it may be difficult to figure out which one it is but it will have a damage value of 9 and won't be a vanilla item (which generally use IDs in the 0-450 range). You can either delete the item entirely or set its damage value to a valid number between 0-8 (you can see a list of block types here, on lines 34-42, and choose the one that corresponds to what you think it should be).
Version 1.7.10
Modpack: regrowth
Crash Report: https://pastebin.com/6B8ujq0K
Specific questions : is it an item in my inventory, or on the ground?
How would I go about fixing an item in my inventory, what file is that information stored.
If the item is on the ground, do I just need to go in and correct it with Mc edit?
This is caused by an item with invalid metadata (the crash is occurring when it tries to get the icon and they did not put a bounds check in; I know this because I have MCP for 1.7.10 and "func_149691_a" corresponds to Block.getIcon), and it is in your inventory since it references "GuiIngame". You can fix this by opening level.dat with NBTExplorer and looking at the player inventory (under Player, Inventory); if it is crashing as soon as you load the world it will be in your hot bar, slots 0-8. Since you are in 1.7.10 items use numerical IDs so it may be difficult to figure out which one it is but it will have a damage value of 9 and won't be a vanilla item (which generally use IDs in the 0-450 range). You can either delete the item entirely or set its damage value to a valid number between 0-8 (you can see a list of block types here, on lines 34-42, and choose the one that corresponds to what you think it should be).
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?